# A simple continuous and fail-safe water-change system



## David Cottle

I set up my planted aquarium in the past year after discovering how much aquaculture has added to the hobby in the 40+ years since I last kept tropical fish. More attention is now paid to water chemistry - of necessity in a planted tank - and I soon wanted continuous water-change, because it just makes sense to me.

I first tried a siphon-drip from the tank combined with conventional auto top-off equipment. I used a float-operated top-up valve and redundant high-level switches with relays operating solenoid valves in the water supply. To be fail-safe, the valves were a normally closed design that had to be energized to stay open in normal operation. I found the solenoids ran too hot, and there was too much hardware in the tank to keep clean. So I went back to the drawing board.

My present design, in use for two months now, has a top-up arrangement that works by siphon, with no moving parts or electricals. I don’t know why this method isn’t in common use - for automatic top-ups at least, if not continuous water change - but I haven’t found it described anywhere. Maybe it _is_ being used and I just haven’t found it?

My setup is shown in the video:


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## Edvet




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## castle

Very good


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## ian_m

Very nice. Like the thinking of how it behaves in all the failure modes.

I would still be worried about not adding dechlorinator to tap water, especially if chloramine is added.

Standard carbon filters do an acceptable job of removing chlorine but have limited or no effect on chloramine. You need chlorplus filter (is one trade name) in order to remove both chlorine and chloramine.

On another forum one guy wiped out £600 of fish when his continuous water change system dosed all his tanks with chloramine'd water. This was due to the water company injecting chloramine due to a burst pipe in the system. His pre-filter only removed chlorine.


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## David Cottle

Thanks for your comments. Cartridges that remove chloramine are available for this filter. I didn't get one of those because the water treatment plant told me they don't use chloramines for disinfection. But that's not a guarantee that they never will - in an emergency, perhaps. I would probably rest easier if I got the other cartridge.


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## dw1305

Hi all, 





David Cottle said:


> because the water treatment plant told me they don't use chloramines for disinfection. But that's not a guarantee that they never will - in an emergency, perhaps.


That has been the problem in the UK. 

Our supply is usually lightly chlorinated (less than 0.5 ppm), but if there is a threat to the integrity of the water main the water companies use emergency chloramine dosing. There was an epidemic of both burst water pipes and fish deaths in early 2010 after some (for us) cold weather. 

It is fairly grizzly reading, but have a look at Shane Linder's tale of woe about the Washington DC water supply in, <"Expert, maybe not....">. 

cheers Darrel


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## zozo

Nice build.. I wonder is it an open top tank? I ask because i have myself a 110 litre open top and it evaporates +/- 1 litre / 24 hours. Than 10 litres p/w would nearly be a top off only replacing what is evaporated. 

And reading all this i realize how spoiled i am. My water company asures us, there are no Cl products in our tapwater and never will. Maybe i pay more i don't know but it;s worth that not having to worry.


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## David Cottle

Yes, it's open top.


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## zozo

Ok i listened again to what you say in the vid.. Language barrier..  You explained what is equal to 10 litres per week but you do a 100 litres p/w. 
I missed the 100 litre watching it for the first time.

Does this also mean you do not use any filtering? Filtering seems redundant with doing about a complete 100% WC p/w.


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## zozo

Btw for your question how to post a vid? It's likely the M dot in the address not recognized by the forum software as a valid video url.

If you look bellow the video in YouTube publicaly after you uploaded it, you see next to the thumbs up and down also a Share button, if you click it a share link pops up, copy that url and put in your repply it always shows without propblems.


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## David Cottle

1 ml/min = 10 L/week (just stating the equivalence)
10 ml/min = 100 L/week
Yes, I still have to filter to remove particulates and also provide circulation through biological media.

Thx for your advice on the video URL. It worked


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## David Cottle

Btw, my purge water siphon tube is located in the outflow from the filter where water should be clearest, and least likely to clog the tubing. Clogging is most likely to occur at the pinch clamp restriction. I look at the purge drip rate regularly, and if it needs adjustment, I periodically open the pinch clamp to flush the tubing before resetting the flow rate.


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## sciencefiction

David Cottle said:


> 1 ml/min = 10 L/week (just stating the equivalence)
> 10 ml/min = 100 L/week



Great system. The only thing I'd mention is that drip system removing 100L a week isn't as efficient as doing a one time 100L water change.

I watched the video but can't remember exactly but I am assuming your tank is 130 litres. 

Your drip system is set to remove 10 ml/min.

Let's round it to 600ml of water being removed every hour. 600ml is 0.6 litres/Hr.
If you are to manually do 0.6 litres water changes every hour, to bring down a pollution of, for example, 40ppm nitrates down to 9ppm, it will take you roughly 313 hours. 313 hours is roughly 13 days of continuous water changes.

If you are to do one 100l water change a week, it will bring the level down to around 9ppm at once.

This result is because when you do smaller water changes, although more often and to the equivalent of one bigger water change, there's a factor of dilution of every 10ml of new water. I am not sure how to explain it but one can calculate it mathematically. So in effect, you're doing one 100l water change every two weeks via the drip system. If you speed it up double that, you'll equal the efficiency of 100l water change a week.


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## David Cottle

Yes, I realize that continuous and batch water-change volumes aren’t equal. A continuous purge removes a mixture of newer water and older water. I think I’ve seen calculators online for determining the continuous water-change equivalent of a specific weekly water change. But I don’t really care about that. I just care about purging impurities (biowaste, etc.) in the aquarium water at a rate sufficient to keep them below harmful or toxic levels.

For any specific continuous water purge rate, the concentration of each impurity will rise or fall until an equilibrium (a.k.a. “steady-state”) concentration is reached, at which the impurity is being purged at the same rate as it’s being generated. Higher purge rates will result in lower equilibrium impurity concentrations; lower purge rates will result in higher concentrations.

Because I can measure nitrate concentration, I’m using it as my bellweather. For the same daily rate of fertilizer addition as I was using with 40-50% weekly water changes, my nitrate levels tend to be lower now than they were with the weekly changes. So I’m comfortable that my present continuous water-change rate is purging impurities as well or better than my weekly changes. I can, of course, increase the purge rate if I wish. With more experience, I might settle on a different purge rate.

Thanks for your thoughtful comments.


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## sciencefiction

David Cottle said:


> I think I’ve seen calculators online for determining the continuous water-change equivalent of a specific weekly water change.



I did mine in excel sheet but sure there are online ones. I just never thought to check, lol. 
I don't ever bother measuring nitrates. I prefer a TDS meter and measure conductivity. If it increases gradually, not enough water changes...


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## David Cottle

sciencefiction said:


> I don't ever bother measuring nitrates. I prefer a TDS meter and measure conductivity. If it increases gradually, not enough water changes...


I'm not that advanced yet


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## sciencefiction

David Cottle said:


> I'm not that advanced yet



You're quite advanced considering your water change regime 

A TDS meter can be bought cheap and it takes 2 seconds to measure as you dip it in the water and it gives you the reading. Its something you may consider getting


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## David Cottle

Yes, I’ll look into it - thanks.


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## dw1305

Hi all,





sciencefiction said:


> I prefer a TDS meter and measure conductivity. If it increases gradually, not enough water changes...


That one.





David Cottle said:


> I'm not that advanced yet


It is actually the other way around, you wouldn't know it from most forums, but nitrate (NO3-) is fairly problematic to measure. Have a look at <"Testing strips vs .....">.

Against that conductivity meters have the advantage of giving consistent results over a large range of values, you get a linear response from RO water ~4 microS all the way up to sea-water at 53,000 microS. You really can just dip the meter in a take a reading, you want a low range meter, "Hanna" is a good make.

All TDS meters are really conductivity meters, they jut use a conversion factor (usually 0.64.) to convert conductivity in microS to ppm TDS.

Cheers Darrel


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## David Cottle

dw1305 said:


> nitrate (NO3-) is fairly problematic to measure



I’m using the API Freshwater test. Haven’t found test strips accurate for anything - at least, they don’t agree with API test results. For nitrate, I can’t see the difference between 10 ppm and 20 on the colour chart, so I dilute the sample to get the concentration in the range of 0-10 and factor the result. This changes the concentrations of other components, which may affect results.

Maybe one day when I have nothing better to do, I’ll prepare some standard solutions and check the accuracy of the API test


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## ian_m

David Cottle said:


> I’ll prepare some standard solutions and check the accuracy of the API test


Well that can be one issue with test kits, repeatability with known strength solutions, in that different manufacturers kits give different readings for the same solution.

The main problem with hobby test kits is ionic interference. Other salts in the tank water can cause test kit false readings. Chloride ions can influence nitrate results, chlorine present in untreated tap water causes a lot of test kits to under read and dechlorinator can cause test kits to over read.

If you insist on wasting time and money testing your water using test kits, then proper "big boy" kits are the way to go. These are generally not affected by other ions.

Nitrate
https://uk.hach.com/nitrate-test-kit-model-ni-11/product?id=26427780260&callback=pf
Phosphate
https://uk.hach.com/test-kit-phosph...del-po-19a/product?id=25114225337&callback=pf
Chlorine
https://uk.hach.com/chlorine-free-c...66f-pk-100/product?id=25114235552&callback=pf
Anyway you get the idea.


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## dw1305

Hi all, 





ian_m said:


> The main problem with hobby test kits is ionic interference.


That is the main problem.

Strangely it is easier with sea water, because you can take into account a <"known amount of chloride ions (Cl-)">, about 19 ppt (1.9% by volume), but in freshwater you have different amounts of anions, dependent upon whether the water is hard or soft etc.

There are a number of issues particularly with the nitrate ion (NO3-), and one of these is that <"nearly all nitrate compounds are soluble">, and if we want to use a colorimetric test (one that uses change of colour), we need the end product (the thing we measure) to be a coloured compound. 

Colour kits work by nitrate reduction to nitrite (NO2-), which forms a number of coloured compounds. The two normal methods are <"cadmium reduction" and "azo dye" formation">. Cadmium reduction has the <"cadmium (Cd) issue">, so the "azo dye method" is usually used (a red colour shows the concentration of NO3, with cadmium reduction you get an amber colour reading).  If you have an amber coloured kit, you really need to shake the bottle hard to make sure you get the full colour development.

You tend to get a slightly more accurate results with an ion selective electrode (ISE), but  even then you can get problems with ion interference (the ion selective bit is a semi-permeable membrane which filters out ions which are the "wrong" size).  Ion selective electrodes are quite expensive bits of kit.

This was a major reason for using <"conductivity and the Duckweed Index"> as a proxy for nutrient status, it isn't perfect but it probably gives you a better idea across a whole range of water conditions.

cheers Darrel


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## David Cottle

ian_m said:


> If you insist on wasting time and money testing your water using test kits, then proper "big boy" kits are the way to go.



That's a pretty broad statement 

When I did a fishless cycling of my new tank, the API ammonia test kit showed concentrations consistent with the amounts of aqueous ammonia I had added to the tank. Subsequent nitrite measurements were consistent with the stoichiometry. Threadfin Rainbowfish added after the test showed zero nitrites did well. The tests apparently served their purpose.

As for the nitrate test accuracy, I appreciate the alarms raised here. I _would_ like an idea of actual nitrate concentrations, and might get a Hach kit if I find that API testing of samples of my (C-filtered) water with measured amounts of my fertilizer solution gives inconsistent results. Don't need absolute accuracy - approximate should do. The Hach test, at $1+ per test, might be overkill for routine monitoring.



dw1305 said:


> Colour kits work by nitrate reduction to nitrite (NO2-), which forms a number of coloured compounds. The two normal methods are <"cadmium reduction" and "azo dye" formation">. Cadmium reduction has the <"cadmium (Cd) issue">, so the "azo dye method" is usually used (a red colour shows the concentration of NO3, with cadmium reduction you get an amber colour reading).  If you have an amber coloured kit, you really need to shake the bottle hard to make sure you get the full colour development.







The pic shows an older API nitrate test bottle #2 that I cut open to have a look inside. I assume the silver-grey residue on the bottom is metallic Cd powder. It's loose - not caked. I taped the bottle closed, shook it vigorously, and found most of the powder in suspension when I reopened the bottle.

A bit of reading of "API nitrate test reviews" indicates that test results are technique-sensitive, which is probably the case with all hobby tests, and perhaps more-so with this one. I suspect that shaking of the final sample-reagent mixture for the time specified is probably more crucial than prolonged shaking of reagent bottle #2 before dispensing - as long as a surplus of Cd is dispensed for the test. I calculate the Cd equivalent of 40 ppm nitrate, e.g., in a 5-ml sample to be 0.36 mg.

If I'm going to check the accuracy of the test with my water, I might as well check results with varied shaking to calibrate my technique.

I'll also get a conductivity meter for additional information and will probably use it eventually for monitoring. At this point, however, I'm not sure that conductivity alone will tell me how much nitrate I have, versus an accumulation of other fertilizer components such as sulphates.

And I'll read up on your Duckweed Index 

Thx.


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## sciencefiction

David Cottle said:


> I'll also get a conductivity meter for additional information and will probably use it eventually for monitoring. At this point, however, I'm not sure that conductivity alone will tell me how much nitrate I have, versus an accumulation of other fertilizer components such as sulphates.



Why do you need to know the level of nitrates? If the conductivity is increasing from the value of your tank water, and you are not adding artificial fertiliser,then water quality is deteriorating. If the nitrate test at the same time has shown low levels of nitrates, and you decrease the water changes amount/frequency because of that, then you're on the wrong path. 

The nitrate test will be inconsistent with the TDS increase, depending on the tank set up, especially in a planted tank. The last time I did a nitrate test was somewhere around 2014. The nitrate in the tank was barely 5ppm.  That's because I had a heavily planted tank at the time in which I didn't dose fertiliser. But I promise you TDS was rising steadily if I don't water change, regardless of levels of nitrates. I once stopped doing water changes for about 5 months, unhappy fish, TDS doubled over that period!


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## dw1305

Hi all, 





David Cottle said:


> The pic shows an older API nitrate test bottle #2 that I cut open to have a look inside. I assume the silver-grey residue on the bottom is metallic Cd powder. It's loose - not caked. I taped the bottle closed, shook it vigorously, and found most of the powder in suspension when I reopened the bottle.
> 
> A bit of reading of "API nitrate test reviews" indicates that test results are technique-sensitive, which is probably the case with all hobby tests, and perhaps more-so with this one. I suspect that shaking of the final sample-reagent mixture for the time specified is probably more crucial than shaking of reagent bottle #2 before dispensing - as long as a surplus of Cd is dispensed for the test. I calculate the Cd equivalent of 40 ppm nitrate, e.g., in a 5-ml sample to be 0.36 mg.
> 
> If I'm going to check the accuracy of the test with my water, I might as well check results with varied shaking to calibrate my technique.


I don't which bottle is which in the API test, but that seems likely, and a good shake should work.





David Cottle said:


> I _would_ like an idea of actual nitrate concentrations, and might get a Hach kit if I find that API testing of samples of my (C-filtered) water with measured amounts of my fertilizer solution gives inconsistent results. Don't need absolute accuracy - approximate should do. The Hach test, at $1+ per test, might be overkill for routine monitoring.


I would like to know as well. 

We are lucky in the UK that we can get accurate NO3 readings from our water supplier, but after that it becomes more problematic. @alto is a scientist and he doesn't see any reason why you can't get accurate results with kits, if you follow the <"proper protocol and the scientific method">.

I'm going to stick with using plant health and colour, it isn't perfect, but it has a <"basis in science">. 

cheers Darrel


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## David Cottle

sciencefiction said:


> Why do you need to know the level of nitrates?



Well, for several reasons. I'm new at this, as well as curious and somewhat analytical by nature. I add fertilizer daily, and am trying to stay within a nitrate range of 5-20 ppm that is good for plant growth but not stressful for fish. If my measurement is at the high end of this range, I'll skip a daily dose; if it's low, I increase the dosage.

With continuous water change and an increase in conductivity, I wouldn't know without other information whether to increase the purge rate or reduce the fertilizer dosage . With enough information and more experience, I'll have a better feel - like you - for how best to regulate it


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## Edvet

David Cottle said:


> trying to stay within a nitrate range of 5-20 ppm that is good for plant growth but not stressful for fish


There are plenty scientific papers where high nitrate levels ( up to 500 ppm) are not detrimental to fish health. However if these high levels are caused by the nitrogen cycle, the higher nitrite and ammonia levels are dangerous. Adding NO3 isn't.


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## sciencefiction

I agree with Edvert. One forgets that in order for a tank to build up nitrates, there must have been sufficient ammonia produced at the same time. And because people test ammonia as zero in a cycled tank, they tend to forget ammonia is actually constantly being produced/converted to less harmful compounds 24/7 in a fish tank and it may spike, whether you happened to test at that exact moment or not.  Go blame high nitrates for your fish problems when in the first place there is high levels of ammonia being produced...whether detected or not......

That process is heavily oxygen demanding(oxygen is a limitation in fish tanks) and the by-products of aerobic and anaerobic nitrification are numerous and mostly harmful. Nitrate in a planted tank is actually never a problem. Plants can also use ammonia directly as nitrogen source, which is one of the great benefits of plants, thus reducing heavy nitrification.


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## dw1305

Hi all, 





Edvet said:


> There are plenty scientific papers where high nitrate levels ( up to 500 ppm) are not detrimental to fish health. However if these high levels are caused by the nitrogen cycle, the higher nitrite and ammonia levels are dangerous.


It is like @Edvet says this is quite an interesting area, because it is only really aquascapers who add NO3 to water containing fish. 

In aquaponics (or organically polluted water etc.) the NO3 is there as the "smoking gun" of  earlier elevated levels of ammonia (NH3) and nitrite (NO2), and usually goes hand in hand with <"elevated levels of orthophosphate (PO4---)">. 

Unfortunately we don't know at what level nitrates become a problem for most ornamental fish, although we could assume that it would be higher for most of these than it would be for <"aquacultured  salmonid fish">. There are some more references in <"What is nutrient toxicity"> and <"Nitrates">





David Cottle said:


> I add fertilizer daily, and am trying to stay within a nitrate range of 5-20 ppm that is good for plant growth but not stressful for fish. If my measurement is at the high end of this range, I'll skip a daily dose; if it's low, I increase the dosage.


 This is back to why I like plants as a visual indicator of nutrient content, you have a negative feedback loop where increased fixed nitrogen leads to increased plant growth, which leads to lower fixed nitrogen levels. All nutrients can be limiting (this is back to <"Liebig's law of the minimum">) but a floating plant takes carbon (CO2) out of the equation and then plants need about an order of magnitude more nitrogen (N) and potassium (K) than phosphorus and about three times as much phosphorus (P) as magnesium (Mg) etc., again there is a more detailed discussion in the <"Nitrate"> thread. 

Personally I like (and have) low nitrate levels. How do I know this if I don't test? Because I have floating plants that don't ever grow very quickly, or ever look brilliant green, and I have low conductivity tank water. If you only have 100 microS of conductivity, you have very few ions of any description. 

cheers Darrel


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## David Cottle

My control range is based on the EI dosing target range of 5-30 ppm, as given here:
http://www.aquaticplantcentral.com/forumapc/fertilizing/15225-estimative-index-dosing-guide.html

I assume high levels make more algae, even if they don't bother the fish?


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## sciencefiction

dw1305 said:


> Personally I like (and have) low nitrate levels. How do I know this if I don't test? Because I have floating plants that don't ever grow very quickly, or ever look brilliant green, and I have low conductivity tank water. If you only have 100 microS of conductivity, you have very few ions of any description.



I sort of "measure" nitrogen the same way, by the condition of the plants, but for me floaters completely die off, each time I tried keeping them. So I could not use the "Duckweed method"  Then again I've also been keeping emersed plants for years, which seem to out-compete floaters for nutrients. Either way, my tanks are constantly nitrogen deficient. There's a couple of pictures I just snapped...


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## dw1305

Hi all, 





David Cottle said:


> I assume high levels make more algae, even if they don't bother the fish?


A bit of an unknown really, advocates of EI will tell you most forcibly that it doesn't. In the natural environment high levels of phosphorus and nitrate are definitely the markers of eutrophication, and often occur together with an algal bloom. 

Physiologically the green algae belong to the same clade (<"Viridiplantae or Chlorobionta">) as all the higher plants and have the same photosystems and basic physiology. Because of this I can't see why conditions that favour the growth of the plants you want, doesn't also favour the growth of the plants you don't want ("algae"). 

Personally I don't mind a bit of algae, in fact I'd actually like a bit more <"periphyton or aufwuchs"> in the tanks.

cheers Darrel


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## David Cottle

dw1305 said:


> Physiologically the green algae belong to the same clade (<"Viridiplantae or Chlorobionta">) as all the higher plants and have the same photosystems and basic physiology. Because of this I can't see why conditions that favour the growth of the plants you want, doesn't also favour the growth of the plants you don't want ("algae").


I've read that heavy planting reduces algae in a planted tank. A common explanation is that the plants out-compete the algae for nutrients - which makes no sense to me whatsoever.

A more plausible explanation is that heavy planting creates conditions that are unfavourable for algae growth, such as postulated here: https://www.ukaps.org/forum/threads/what-exactly-causes-bba-part-2-bacterial-imbalance.38375/
(haven't figured out how to create a link yet )


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## sciencefiction

David Cottle said:


> I've read that heavy planting reduces algae in a planted tank. A common explanation is that the plants out-compete the algae for nutrients - which makes no sense to me whatsoever.



I have personally seen my iron deficient green spot algae come to life once I dosed iron. It had gotten as pale as the plants I was trying to fix. I've no idea who out-competed who for iron


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## dw1305

Hi all, 





sciencefiction said:


> I have personally seen my iron deficient green spot algae come to life once I dosed iron.


The algae can show a rapid response to iron addition, if iron is deficient, because it has no internal plumbing and  Fe++(+) ions will diffuse into each cell. 

Higher plants won't show that instant response, because they can't transport the iron to already grown tissue, it can only effect the new leaves developed after iron was non-limiting. 





David Cottle said:


> I've read that heavy planting reduces algae in a planted tank. A common explanation is that the plants out-compete the algae for nutrients - which makes no sense to me whatsoever. A more plausible explanation is that heavy planting creates conditions that are unfavourable for algae growth,


The real answer is we don't know how heavy planting works, but it definitely does reduce algae in low tech tanks. 

cheers Darrel


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## zozo

David Cottle said:


> A common explanation is that the plants out-compete the algae for nutrients - which makes no sense to me whatsoever.



That's also a freedom of (mis)interpretation..  if the same sentence was put more to the point with "That enough healthy growing plants out-compete algae for nutrients"Would already make a lot more sense, it's a logical sum. Still doesn't say that if you have enough healthy growing plants you never will grow algae.


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## David Cottle

dw1305 said:


> conductivity meters have the advantage of giving consistent results over a large range of values, you get a linear response from RO water ~4 microS all the way up to sea-water at 53,000 microS. You really can just dip the meter in a take a reading, you want a low range meter, "Hanna" is a good make.


Darrel, is 0-1999 microS/cm a suitable range? And how often do you calibrate these?


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## dw1305

Hi all, 





David Cottle said:


> Darrel, is 0-1999 microS/cm a suitable range? And how often do you calibrate these?


Yes that is fine, you won't get above 1999 microS. 

They differ from pH meters in that they only need re-calibration fairly infrequently (every three months or so) and you can make up the <"calibration standard your-self">, from potassium chloride (KCl) 0.01 M  = 1411 µS at 20oC. I make up a 0.1M  KCl solution and then dilute it (100 cm3 0.1M KCl made up to 1000cm3 with DI water)

When you want to take a reading you turn the meter on, dip it in the tank water and give it a swirl and take the reading.  

cheers Darrel


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## David Cottle

dw1305 said:


> They differ from pH meters in that they only need re-calibration fairly infrequently (every three months or so) and you can make up the <"calibration standard your-self">, from potassium chloride (KCl) 0.01 M = 1411 µS at 20oC. I make up a 0.1M KCl solution and then dilute it (100 cm3 0.1M KCl made up to 1000cm3 with DI water)


Great!

I have KNO3 (and some other salts from Green Leaf Aquariums), not KCl. I can probably get KCl at the pharmacy.

Thanks


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## dw1305

Hi all,





David Cottle said:


> I have KNO3 (and some other salts from Green Leaf Aquariums), not KCl. I can probably get KCl at the pharmacy.Thanks


 They sell KCl as "sodium free salt" in the UK, but it isn't always pure KCl (it often has MgCO2 as dessicant), and may even contain some NaCl.

I'm not sure if these sell to Canada (it is a USA based company, and I don't know anything about them), but this sort of thing <"Now Foods - potassium chloride powder">.

If you can get pure KCl it is 0.746g KCl in one litre to give 1411 microS. I keep the 0.1M KCl stock (7.46g in 1000cm3) in a stoppered bottle, but I only use the 1:10 diluted calibration solution once, and I use the whole large volume for calibration. The reason for this is that they aren't like pH buffers, any drips of lower conductivity water will dilute the standard and change the microS reading.

I found this out the hard way when I bought some combined pH/Conductivity meters for field work. The same probe measured both pH and conductivity and you store the electrode in 4M KCl. Even the smallest trace of this storage solution left on the probe meant that you couldn't calibrate the meter for conductivity, and these particular units required calibration every time they were turned on, because they were pH probes.  

If you can get pure NaCl? You need 0.491g (491mg) L-1 NaCl to give a 1000 µS standard.

cheers Darrel


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## David Cottle

Thanks Darrel


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## David Cottle

dw1305 said:


> When you want to take a reading you turn the meter on, dip it in the tank water and give it a swirl and take the reading.


OK, my tap water reads 346 uS and my tank is 442. What does that say about my water?


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## sciencefiction

David Cottle said:


> OK, my tap water reads 346 uS and my tank is 442. What does that say about my water?



The difference could be due to fertilisers, if you've added any. If not, tap and tank should read about the same after the weekly water change. The frequency of water changes should be often enough not to let any significant rise. The reading would naturally rise up till the next water change but the water changes should be enough to bring the reading back to tap water each week. I am pretty certain you'll have healthy fish that way. If you fertilise, you can take a reading after its well mixed up to get an idea what effect it has but regardless, you should bring the TDS back to tap water in the end of the week.

Basically, it doesn't matter what your tap reading is. You can't change that. My tap water reading is rather consistent within 5ppm. What matters is the rise from water change to water change.


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## David Cottle

sciencefiction said:


> The difference could be due to fertilisers, if you've added any.


I add fertilizers each morning. Will check before and after.

Don't know why tap and tank should be the same after, say, a 50% weekly water change (that I no longer do). Why wouldn't the tank after the change be somewhere in the middle of tap and the tank before a 50% change (since you're keeping half of the electrolytes)?


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## sciencefiction

David Cottle said:


> I add fertilizers each morning. Will check before and after.
> 
> Don't know why tap and tank should be the same after, say, a 50% weekly water change (that I no longer do). Why wouldn't the tank after the change be somewhere in the middle of tap and the tank before a 50% change?



It may not be, depending on your stock level, feeding, fertilisers, etc.... But my advice is, you should make it happen if you want healthy, long lived fish 

My tank's TDS barely rises week from week. I haven't dosed anything in months. It is lightly stocked. I do 50-70% water change each week. I go by the water level when I water change, and always go below what I think is 50%. In my shrimp/hillstream tank I do more like 80% weekly, fish barely have room to swim. Its a small tank so water quality is more difficult to maintain unless one does enough water changes.


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## dw1305

Hi all,





David Cottle said:


> OK, my tap water reads 346 uS and my tank is 442. What does that say about my water?


It just says you have more salts in your tank water then your tap water.





David Cottle said:


> I add fertilizers each morning.


Almost certainly the fertilizer.

Measure conductivity again next week, if it keeps creeping up over the next four weeks or so change a bit more water.

cheers Darrel


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## sciencefiction

I'd say roll back the tank via a couple of heavy water changes to tap water and go from there. No need to "build" up your experiments on tank water that is already roughly 65ppm over your tap measurement(if I am calculating correctly)


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## David Cottle

sciencefiction said:


> No need to "build" up your experiments on tank water that is already roughly 65ppm over your tap measurement(if I am calculating correctly)


65 ppm TDS seems consistent with EI Dosing target nutrient concentrations of NO3 5-30ppm, K 10-30ppm, CO2 30ppm, etc. The PPS-Pro solution recipe I'm using from Green Leaf Aquariums includes K2SO4 and MgSO4, contributing additional conductive "solids". These levels might seem high to you because you're not dosing?


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## sciencefiction

David Cottle said:


> These levels might seem high to you because you're not dosing?



Yes, perhaps.
The idea is that the plants use up what one doses. In reality most people overdose and get a build up. Roll forward a few weeks/months and you could easily have 130ppm on top of tap water...then why fish don't last.... If it were me, each weak would be a starting point.  If I got 65ppm more at the end of that week, I am overdosing.
The only stuff that would raise your TDS to that point is KNO3. The rest are dosed in such small amounts/ppm term wise, that they don't count. But my guess is you're not dosing 65ppm in nutrients, because one should also consider plants nitrogen consumption. So you're either dosing too much, or tank waste is building up, or both.  You know, its not that easy to "water change" 65ppm completely. You'd need to do several back to back water changes to remove those extra "nutrients" ....but it is very easy building them up in the space of a week...


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## David Cottle

sciencefiction said:


> The only stuff that would raise your TDS to that point is KNO3. The rest are dosed in such small amounts/ppm term wise, that they don't count. But my guess is you're not dosing 65ppm in nutrients, because one should also consider plants nitrogen consumption. So you're either dosing too much, or tank waste is building up, or both. You know, its not that easy to "water change" 65ppm completely. You'd need to do several back to back water changes to remove those extra "nutrients" ....but it is very easy building them up in the space of a week...


The fertilizer recipe I'm using has almost as much K2SO4 as KNO3, plus 2/3 as much MgSO4, all of which will contribute to TDS, plus minor amounts of other constituents. The reason they use a seemingly odd combination of N-P-K salts is apparently to achieve NO3/PO4/K proportions of precisely 10/1/10 by weight (_I checked it _). I've forgotten the reason for the Mg.

I shouldn't be "building up" over the course of a week. The whole idea of continuous water change is to keep everything - wastes, nutrients, contaminants - constant at good levels.

Re an earlier comment from you about water-change efficiency, I didn't answer it very well. If I wanted to drop waste/contaminant concentrations substantially, I would do a batch water change, which is efficient for this purpose. But if I want to keep water quality steady and not ever reach high waste/contaminant levels, continuous water change is the efficient way to do this.


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## sciencefiction

David Cottle said:


> The whole idea of continuous water change is to keep everything - wastes, nutrients, contaminants - constant at good levels.



Not really, your "continuous" water change is equivalent to a 50% water change weekly. Its up to you how to keep your tank. I personally would not take lightly 65ppm rise on top of my water as a long term solution/normal thing, unless it really stays rock solid around that measure.



David Cottle said:


> I shouldn't be "building up" over the course of a week.



"Shouldn't" is doing the guessing. That's why I suggested the TDS meter in the first place. You get the "guessing" out of the equation. In fact, a lot of people out here that got TDS meters for the first times tend to measure quite higher TDS than that of their initial tap water, even double and treble amounts....They too were guessing....Some used to use nitrates tests, other went by the "heavy planting" theory....There's nothing wrong with that but neither of those methods are reliable in terms of finding out what one's water quality is.


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## sciencefiction

There is another thing I forgot to mention. I speak from fish's perspective. To me fish are first and foremost, plants don't matter. In fact at the moment I am not really a plant keeper unless you count what grows without any effort on my part. You may have a different point of view as many plant keepers tend to do...

When there's more nutrients in the tank, the plants will be happier. Fish health does not always correlate with plant health though....


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## David Cottle

sciencefiction said:


> I personally would not take lightly 65ppm rise on top of my water as a long term solution/normal thing, unless it really stays rock solid around that measure.


I deliberately add fertilizers every day to maintain this rise. Doesn’t everyone who doses fertilizers get such a rise? Conductivity monitoring will tell me how steady it is.


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## sciencefiction

David Cottle said:


> I deliberately add fertilizers every day to maintain this rise. Doesn’t everyone who doses fertilizers get such a rise? Conductivity monitoring will tell me how steady it is.



Absolutely everyone gets a TDS rise at the end of the week.....The question is how much and what drives the rise, and do you do anything about it...., and what do you do about it..


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## dw1305

Hi all, 





David Cottle said:


> I've forgotten the reason for the Mg.


It is the central atom of the chlorophyll molecule. 



 
Because of <"where you live"> you may well have magnesium in your tap water, in the UK we don't tend to. 





David Cottle said:


> I deliberately add fertilizers every day to maintain this rise. Doesn’t everyone who doses fertilizers get such a rise? Conductivity monitoring will tell me how steady it is.


That is it. When the plants are healthy and the tank is running well you have a datum range for conductivity.

Because I use rain-water, the duckweed index and have low nutrient tanks I use the 80 - 140 microS datum.  If the tank gets below this, but the plants look healthy, I add some tap water (about 17 dKH). If the tank gets a higher, I add some more rain-water or RO. Because locally is all limestone our rain water has some dKH, presumably from dust it picks up. 

If I used <"our tap water">, I'd use the same approach, but my initial datum would be much higher, because the tap water starts at about ~600 microS from all the dissolved limestone.

cheers Darrel


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## David Cottle

dw1305 said:


> It is the central atom of the chlorophyll molecule


_I knew that!! _(at least, I knew it was in there somewhere ) but I forgot, thanks. The caprock around here is dolomitic limestone, so there's probably enough Mg in the tap water to begin with.


sciencefiction said:


> Absolutely everyone gets a TDS rise at the end of the week.....The question is how much and what drives the rise, and do you do anything about it...., and what do you do about it..


I don't have an end of the week. Every day is the middle of the week in my tank. What I do about controlling TDS is purge continuously, 24/7. Don't know what else to say - we just operate our tanks differently, and we have different TDS targets. Not sure what my target, expressed as ppm TDS, is exactly - but I'll know soon.


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## sciencefiction

David Cottle said:


> _I knew that!! _(at least, I knew it was in there somewhere ) but I forgot, thanks. The caprock around here is dolomitic limestone, so there's probably enough Mg in the tap water to begin with.
> 
> I don't have an end of the week. Every day is the middle of the week in my tank. What I do about controlling TDS is purge continuously, 24/7. Don't know what else to say - we just operate our tanks differently, and we have different TDS targets. Not sure what my target, expressed as ppm TDS, is exactly - but I'll know soon.



I know you don't have "end of the week" but I think you misunderstand me. Your water's measure, whether in EC or TDS units, has already increased its value from base tap water. And exactly because your tank operates under "continuous" water change, if all 65ppm are from your ferts, you're way over-fertilising. What would plants need at any given time/snapshot in time?...5-10ppm Nitrates, a few ppm Potassium and the lot of the rest wouldn't exceed 5ppm, so say generously say 10- 20ppm in ferts...Your tank could easily be "polluted" right now, whether that's from in-organic fertilisers or organic ones, produced by nitrification....


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## a1Matt

First off, I appreciate you posting your WC setup, it looks beautifully engineered.

The pertinent point (IMO) with regards to TDS is to observe the trend, i.e. is it rising, falling or staying steady.

Then once you know that you can choose what to do next.

If it is rising, it could be either from excess ferts, fish waste or uneaten food.

One simple way to gauge it is to simply stop adding ferts for a week.

I'm sure you knew that already, so the main point of this post is to let you know that if the plants are healthy and well fed then they can go without ferts for a week without any issues (and possibly a lot longer depending on plant biomass vs fish waste and the light levels, but how much longer I wouldn't like to guess over the internet).


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## David Cottle

sciencefiction said:


> I know you don't have "end of the week" but I think you misunderstand me. Your water's measure, whether in EC or TDS units, has already increased its value from base tap water. And exactly because your tank operates under "continuous" water change, if all 65ppm are from your ferts, you're way over-fertilising. What would plants need at any given time/snapshot in time?...5-10ppm Nitrates, a few ppm Potassium and the lot of the rest wouldn't exceed 5ppm, so say generously say 10- 20ppm in ferts...Your tank could easily be "polluted" right now, whether that's from in-organic fertilisers or organic ones, produced by nitrification....


You're ignoring my earlier responses to you showing that 65ppm is consistent with EI Dosing target nutrient ranges, not even counting MgSO4 or sulphates in K2SO4 that is part of the PPS-Pro fertilizer solution recipe I use.

Following the PPS-Pro guide, I add 240 mg NO3 daily. That equates to slightly less than 2 ppm NO3 added per day in my 130L tank. 240 mg NO3/day x 7 = 1680 mg NO3/wk.
The EI Dosing guide for a 20-40 U.S. gal. tank calls for addition of 800 mg NO3 three times/wk. = 2400 mg NO3/wk, so I'm well below that.

At the same time, I'm purging 10 ml/min = 14.4L/day x 10 mg NO3/L est. = 144 mg NO3/day, i.e., I'm continuously purging more than half of what I add, so my net addition is less than 1 ppm/day. And everything appears to be stable.



a1Matt said:


> The pertinent point (IMO) with regards to TDS is to observe the trend, i.e. is it rising, falling or staying steady.Then once you know that you can choose what to do next.


I've been doing that with the API nitrate test for over 3 months of steady operation, and haven't seen a rising trend. The fish are doing fine. I've just started measuring conductivity following suggestions in this thread, so I'll have additional information to track.

A nice thing about continuous water change is that it‘s largely self-regulating. If NO3 concentration starts to rise, e.g., the amount purged rises accordingly. At 15 ppm NO3 in the tank, purging will be 50% greater; at 20 ppm, purging will exceed what I'm adding (and I'm pretty sure the plants consume more than what is produced biologically). For every purge rate I choose to use, and assuming a constant rate of nutrient addition, nutrient and contaminant levels will equilibrate at levels where the amounts removed equal the net amounts generated.


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## David Cottle

I've calculated the TDS/EC rise that should result from adding sufficient nutrient solution to achieve my target nutrient range. My "macro" nutrient solution uses the PPS-Pro recipe, which contains the following salts in 500 ml of solution (I'm using distilled water):
KNO3      32.6g      NO3 content = 32.6 x 62/101 = 20.0g
K2SO4    29.3g
KH2PO4   2.9g
MgSO4   20.2g
Total          85.0g

My target NO3 concentration is 10 - 20 ppm. The total PPS-Pro salt equivalent of 10 ppm NO3 = 10 x 85g/20g = 42.5 ppm by wt. The corresponding EC rise = 42.5/0.64 = 66.4 uS.
10 - 20 ppm NO3 = 42.5 - 85 ppm TDS
Corresponding EC rise = 66 - 133 uS
Micro nutrients are additional.

An EC rise of 100 uS puts me right in the middle of my calculated target range, equivalent to 15 ppm NO3.
Stopping fertilizer addition for two days has dropped my EC by 40 uS, to 402 uS this morning.

BBA was a vexing problem in my tank during my first (learning) year. It's completely gone under my present regimen. To borrow a phrase from one of George Farmer's videos, my fish are happy, my plants are happy,
I'm happy .  As I imagine there are still some BBA spores lurking around, and I don't want to deviate too much from plant growth conditions unfavourable to BBA, I'm resuming my dosing .

(Apologies to anyone who read an unrelated message in which I was testing creating a hyperlink - still haven't figured out how to do that, at least it doesn't preview properly - then was unable to edit my test message with this message. Still learning )


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## sciencefiction

David Cottle said:


> My target NO3 concentration is 10 - 20 ppm.



Is this concentration your weekly target or daily target? As far as I know one doses every day or every other day, in order to achieve 10-20ppm total per week. Therefore at any given time, providing your continuous water changes, you want 10-20ppm divided by 7 days? A TDS or EC measure is a snapshot in time. It is the concentration there and then and it does not get any lower than that as time goes by.....



David Cottle said:


> The total PPS-Pro salt equivalent of 10 ppm NO3 = 10 x 85g/20g = 42.5 ppm by wt.



What you're calculating above is not very clear to me. You have:

20g NO3 in 500ml distilled water solution.

You want to achieve 10-20ppm concentration in the tank itself. I will assume you want that measure constant every day, not a cumulative effect....? Either way, for the purposes of calculation I assumed you want that daily....though that's to be disputed...

10ppm = 10 mg/l

If tank is 130 litres, then to achieve 10ppm NO3, you'll need 10*130/130 mg/l = 1300 mg/l = 1.3 g NO3 for your 130 litre tank.

In the given scenario you have 20g NO3 in a 500 ml solution, so you roughly need to dose 500/(20/1.3) ml to achieve 10ppm NO3, which is about 32.5 ml dose from the solution. If you spread that amount over the course of 7 days, it is 4.6 ml a day.

Do you dose 32.5 ml daily or do you dose 4.6 ml daily, or thereabouts?



David Cottle said:


> The total PPS-Pro salt equivalent of 10 ppm NO3 = 10 x 85g/20g = 42.5 ppm by wt. The corresponding EC rise = 42.5/0.64 = 66.4 uS.
> 10 - 20 ppm NO3 = 42.5 - 85 ppm TDS
> Corresponding EC rise = 66 - 133 uS



I am not sure how you arrive at the above figures? In my view:

10 ppm NO3 @ 25C is 10/0.64= 15.64 EC
20 ppm NO3 is 20/0.64 = 31. 25 EC



David Cottle said:


> OK, my tap water reads 346 uS and my tank is 442. What does that say about my water?



Let's say 20ppm NO3 is your daily max amount you want to allow for in your TDS/EC reading and you have an increase from 346 to 442= 96 EC, which by the way is an increase in EC(TDS) of 27.75% !
.
For the purposes of better understanding I will use both EC and TDS values...

96 EC is the equivalent of roughly 61.44 ppm, from which at values for

20ppm NO3 - only 31.25 EC from the 96 EC rise

10ppm NO3 - only 15.64 EC from the 96 EC rise

I am pretty certain PPS-Pro recommends the 10-20ppm as the weekly measure and not daily, therefore if you are to achieve max 20ppm NO3 per week, your daily differential from base tap water to account for the daily NO3 dose, assuming plants used none of it at all, is 31.25 EC/7 days=4.46 EC on top of your base water, or your EC reading should read 346+4.46=350.46.

Which means you have 96-4.46=91.54 EC unaccounted for in you daily EC test, give and take the other ferts and plant uptake...

NO3 is the fert at highest concentration...So again, I think you're either severely overdosing or there's more than just in-organic fertiliser buildup in your tank....


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## David Cottle

sciencefiction said:


> Is this concentration your weekly target or daily target? As far as I know one doses every day or every other day, in order to achieve 10-20ppm total per week. Therefore at any given time, providing your continuous water changes, you want 10-20ppm divided by 7 days? A TDS or EC measure is a snapshot in time. It is the concentration there and then and it does not get any lower than that as time goes by.....


With continuous W/C, the goal is to have water conditions constant from day to day, including the NO3 concentration (my target is 10-20 ppm), TDS and EC.


sciencefiction said:


> What you're calculating above is not very clear to me.


I'm not trying to calculate how much solution I need to get to 10-20 ppm - that's a different matter that isn't relevant to what I'm trying to explain. You say 65 ppm TDS in my tank is way too high. I'm explaining why it isn't, by showing what the TDS will be corresponding to 10 ppm NO3. The ratio of 85/20 is the ratio of TDS to NO3 in the PPS-Pro solution I use. For a concentration of 10 ppm NO3, the TDS that comes along with it wiil give a TDS concentration of 10 x 85/20 = 42.5 ppm. EC = 42.5/0.64 = 66.4 uS. Multiply by 2 for 20 ppm NO3.

Considering _how much_ solution I need to get there is a different matter altogether, and trying to calculate it as you're doing just confuses the discussion. The PPS-Pro guide I follow is given here (I found the link icon ). I adjust the amount of solution added if/as needed to stay in my target range.


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## sciencefiction

David Cottle said:


> Considering _how much_ solution I need to get there is a different matter altogether, and trying to calculate it as you're doing just confuses the discussion.



No, it doesn't. By doing that I am trying to explain a vital point you're missing in your calculation.



David Cottle said:


> the TDS that comes along with it wiil give a TDS concentration of 10 x 85/20 = 42.5 ppm



Your calculations are incorrect. Let's say you're dosing 32.5 ml from a solution with concentration "X". This does not achieve the concentration "X" in the target tank itself but way, way smaller because the tank volume is way way bigger... Your tank is diluting whatever mg/l(ppm) you're dosing into 130 litres of water.....See calculations below.



David Cottle said:


> 10 x 85/20 = 42.5 ppm



The 42.5 value you've calculated is not the concentration of NO3 in the solution or tank for that matter. 85/20 is the reverse ratio between NO3 in grams and total ferts in grams. Then you're multiplying that figure by 10 to achieve....??

 The solution has 20G of NO3 per 500ml of distilled water. In other words that's 40g NO3 per litre which converted in ppm is 40,000 mg/l  or 40,000 ppm , nof 42.5ppm!

If you're to dump the entire bottle of the solution in one go in a tank with a volume of 130 litres, you'd achieve a concentration of 40,000/130=307.7ppm NO3 but since you only want 10ppm, you should only dose 1/30th of that solution at a time. 1/30th x 500ml is 16.7ml. If you want to achieve 20ppm, then that's roughly 33.4 ml or thereabouts.

Point is, whatever the concentration X in the solution, which in this case is 40,000ppm, it gets diluted in the tank, and depending what concentration in the tank you want to achieve, you dose a certain amount of the way more concentrated solution.

However, 10ppm concentration in the tank itself always corresponds to 10ppm and, as per the above in EC, that's about 15 EC..


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## sciencefiction

David Cottle said:


> With continuous W/C, the goal is to have water conditions constant from day to day, including the NO3 concentration



Yes, I completely understand this. What I am trying to say is, aim at lower EC/TDS as a constant than what you have right now...because you're way above the base line, i.e tap water and you need less ferts, as per your own target, than what you have at the moment, assuming all the 96 EC difference is ferts, which I highly doubt is the case.....


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## David Cottle

sciencefiction said:


> Your calculations are incorrect.


I don’t think so. They might be _inaccurate_, to the extent that different constituents in my fertilizer solution may be taken up by the plants or otherwise depleted at different rates, but my guess is that this would _increase_ the added TDS/NO3 ratio in the tank over that in the fertilizer solution if anything.

The fact that the measured EC rise is in close agreement with my calculated values would seem to support my calculations and conclusions.


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## sciencefiction

David Cottle said:


> They might be _inaccurate_,



I think you have not tried to read any of what I wrote. Your calculations are not just inaccurate but completely wrong. However, as long as you're happy with your results, that's what matters because in the end, one can't calculate plant health or fish health. We can only use technical data as a guide, not as ultimate truth.


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## David Cottle

sciencefiction said:


> I think you have not tried to read any of what I wrote. Your calculations are not just inaccurate but completely wrong.


Yes, I read everything you wrote. I don't know why you want to make something so simple into something so complicated. I'll try one more time.

The object is to determine how much TDS is added when I dose with enough fertilizer solution to reach 10 - 20 ppm of NO3.

The TDS will be more than 10 - 20 ppm, because the solution has other components besides NO3, and these count in the TDS measurement.

The ratio of total "solids" (electrolytes, i.e.) to NO3 in my fertilizer solution by weight is 85/20 = 4.25. This means there are 4.25 parts of total solids for 1 part of NO3.

No matter how much of the solution I add, or how much it's diluted in my tank, the ratio of total solids to NO3 will stay the same (except to the extent that different ions may be assimilated by the plants at different rates to change the ratio).

_Therefore, adding enough fertilizer to raise the NO3 concentration in my tank by 1 ppm will raise TDS by 4.25 ppm._

10 ppm NO3 = 42.5 ppm TDS. 20 ppm NO3 = 85 ppm TDS.

It's too simple to calculate incorrectly.


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## sciencefiction

David Cottle said:


> It's too simple to calculate incorrectly.


 
I am tired of explaining so I will try one more time.

TDS  is a measure expressed as either parts per million(ppm) or mg/l. You cannot divide grams by grams


David Cottle said:


> It's too simple to calculate incorrectly.



Not as simple apparently....

I'll try explaining one more time and then I am done.

TDS is a measure you can express in ppm or mg/l where 1 ppm=1 mg/l



David Cottle said:


> The ratio of total "solids" (electrolytes, i.e.) to NO3 in my fertilizer solution by weight is 85/20 = 4.25. This means there are 4.25 parts of total solids for 1 part of NO3.



As explained above TDS(ppm) is a measure of total dissolved solids (in grams) divided by the volume of the solution the solids are dissolved in (in litre)

Therefore your solution of 85 grams solids to 500 ml solution has a TDS of 85 grams/ 0.5 litres. To convert in mg/l=ppm that's 170,000 mg/l .
The TDS of your solution is therefore 170,000 ppm!!!! not 4.25ppm.....

Do you understand where you are going wrong? You are dividing grams by grams and arriving at ppm!! Impossible to calculate TDS like that, which is a measure of milligrams per litre.

What you are doing is calculating that if 85g is the total grams fertiliser and 20g of that is NO3 then the concentration of NO3 in the given 500ml solution is 4.25 titmes less compared to the total grams of fertilisers. For a known TDS(ppm), which in this case is 170,000 ppm, you can then calculate using your "favourite" ratio  that the NO3 concentration in the solution is 170,000/4.25=40,000 ppm, as already previously explained several times.

Now, you DO need to take into account the volume of water of the tank and the amount dosed from the concentrated solution to see how that concentration dilutes and what ppm NO3 it will give in the tank.....as I already calculated several times in different ways....

However, once you've figured how much of the concentrated solution to dose in the tank to arrive at 20ppm, the TDS increase is simply...20ppm! Of course I am not taking into account other factors, but neither are you in your calculations....



David Cottle said:


> 10 ppm NO3 = 42.5 ppm TDS. 20 ppm NO3 = 85 ppm TDS.



10 ppm cannot equal 42.5 ppm and 20 ppm cannot equal 85 ppm

It's like saying that 10=42.5 and 20=85. !!!!

When you have achieved 10 ppm or 20 ppm concentration of NO3 in the tank, then that's the increase theoretically.....as simple as that


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## David Cottle

To resolve this tedious argument, this morning I prepared a solution containing 15 ppm NO3 + micronutrients by taking 5 ml of my PPS-Pro 'macro' solution + 0.5 ml of 'micros' and diluting to 200 ml with tap water; then diluting 15 ml of that solution to 1 L with tap water.

The EC of that solution measures at 428uS. My tap water is 344uS.

My aquarium this morning was 407uS before addition of 6 ml of ferts + micros, and 419 uS afterwards.

QED. Real science works.

The fish are fed and happy .


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## dw1305

Hi all, 





David Cottle said:


> My aquarium this morning was 407uS before addition of 6 ml of ferts + micros, and 419 uS afterwards.


 That is why I like conductivity (in this case expressed as ppm TDS) as a measurement. It doesn't tell you anything specific about the ratio of ions , but it is quick and easy to measure.  





a1Matt said:


> The pertinent point (IMO) with regards to TDS is to observe the trend, i.e. is it rising, falling or staying steady.
> 
> Then once you know that you can choose what to do next.
> 
> If it is rising, it could be either from excess ferts, fish waste or uneaten food.
> 
> One simple way to gauge it is to simply stop adding ferts for a week.
> 
> I'm sure you knew that already, so the main point of this post is to let you know that if the plants are healthy and well fed then they can go without ferts for a week without any issues





dw1305 said:


> That is it. When the plants are healthy and the tank is running well you have a datum range for conductivity.
> 
> Because I use rain-water, the duckweed index and have low nutrient tanks I use the 80 - 140 microS datum. If the tank gets below this, but the plants look healthy, I add some tap water (about 17 dKH). If the tank gets a higher, I add some more rain-water or RO. Because locally is all limestone our rain water has some dKH, presumably from dust it picks up.
> 
> If I used <"our tap water">, I'd use the same approach, but my initial datum would be much higher, because the tap water starts at about ~600 microS from all the dissolved limestone.


cheers Darrel


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## sciencefiction

David Cottle said:


> To resolve this tedious argument, this morning I prepared a solution containing 15 ppm NO3 + micronutrients by taking 5 ml of my PPS-Pro 'macro' solution + 0.5 ml of 'micros' and diluting to 200 ml with tap water; then diluting 15 ml of that solution to 1 L with tap water.
> 
> The EC of that solution measures at 428uS. My tap water is 344uS.
> *
> My aquarium this morning was 407uS before addition of 6 ml of ferts + micros, and 419 uS afterwards*



He, he. I can't help myself 

That's correct.

Your 500 ml solution has a concentration of total ferts of 170,000 ppm

If you add 6ml to a 130 litre that that will give you 6/130,000*170,000ppm =7,846ppm increase in the tank.

Converted to EC that's 7,846/0.64 = 12.26 EC.

Your EC moved from 407 uS to 419 uS, the difference is roughly 12 uS as per the above.

It would be interesting to measure the EC tomorrow, just before you dose the next dose, perhaps to see plant uptake between water changes....



David Cottle said:


> The fish are fed and happy



Glad to hear that


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## David Cottle

sciencefiction said:


> Your 500 ml solution has a concentration of total ferts of 170,000 ppm


I find it easier to think in terms of 40 mg NO3/ml (and TDS 170 mg/ml, which I don't normally worry about - except when I'm arguing with you ). I already know how many ppm NO3 6 ml adds to my tank.


sciencefiction said:


> If you add 6ml to a 130 litre that that will give you 6/130,000*170,000ppm =7,846ppm increase in the tank.


6 ml x (170 mg TDS/ml) /130 L = 7.8 mg/L = 7.8 ppm TDS increase.

Full disclosure: I'm a chemical engineer and have been doing such calculations all my life .


sciencefiction said:


> Do you understand where you are going wrong? You are dividing grams by grams and arriving at ppm!! Impossible to calculate TDS like that, which is a measure of milligrams per litre.


FYI, g/g is a dimensionless ratio. Multiplying ppm NO3 by g TDS/g NO3 gives ppm TDS. Dimensional analysis helps these computations.


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## David Cottle

sciencefiction said:


> It would be interesting to measure the EC tomorrow, just before you dose the next dose, perhaps to see plant uptake between water changes....


EC dropped from 424 to 402 over the two days I stopped dosing early in the week. It all seems pretty stable, considering that my purge rate may drop a little over time and I readjust it, usually by eyeballing the drips, so the change rate fluctuates within a range. I'll also adjust fertilizer dosage if/as needed to stay in a range.


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## sciencefiction

David Cottle said:


> FYI, g/g is a dimensionless ratio. Multiplying ppm NO3 by g TDS/g NO3 gives ppm TDS. Being proficient in dimensional analysis makes computations easier.



Yes, I agree. But you know what I meant by 85/20*10ppm not giving you the concentration of the solution...because it didn't take into account the volume of the solution or the volume of the tank later on. .



David Cottle said:


> I find it easier to think in terms of 40 mg NO3/ml (and TDS 170 mg/ml, which I don't normally worry about - except when I'm arguing with you



 I call it a dispute, not an argument 

Yes, it seems easier to express in mg/ml.. My long winded explanations is a habit from 25 years back. I was in Maths High School and those days calculators of any kind  were not allowed in class...Obviously we had no computers or phones either to be able to cheat.The kids today have it easy...We wrote down everything on paper as you go along with your calculation and it was compulsory to show each line of the calculation, regardless of how easy it is.  I can flip sides of equations to caclulate unknowns in my head and get the answer without a calculator or paper but by habit, if I am asked to I write it down, I'll give you each simplification on a separate line....the long way.....

Those days we calculated everything manually on paper, including logarithms, sinus, cosinus, tangens and the likes. So the long way was the more accurate way  Now you can just press a button on your calculator, lol  I am an accountant by education by the way but I work in finance.  Error prevention, error detection and impact, represents pretty much the bulk of my daily job..I release calculated "numbers"/values of complex financial securities on the market daily, which calculations can have tens to hundreds of pages of paper showing the calculations and verifications,  so I've got to think twice before cutting down 3 zeros from a number


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## Edvet

sciencefiction said:


> I am an accountant by education by the way but I work in finance. Error prevention, error detection and impact, represents pretty much the bulk of my daily job..I release calculated "numbers"/values of complex financial securities on the market daily, which calculations can have tens to hundreds of pages of paper showing the calculations and verifications, so I've got to think twice before cutting down 3 zeros from a number


That's all sciencefiction to me


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## zozo

Edvet said:


> That's all sciencefiction to me



This whole topic is.. Very interesting but siencefiction..


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