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TDS Rise from topping off.

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May be a stupid question which I'm over complicating but why not. Am I right in thinking that if I have a 50 ltr tank with a TDS of 127 which loses about 4 ltrs of water per week and I top this water off with rain water with a TDS of 12 that I've raised the TDS by 12 divided by the 50 ltrs 0.24?

Reason I ask is I'm trying to get a rough idea of fert build up in a low tech, I seem to be getting some consistent results using a rule of third with this tank and good plant growth. I.E I change a third of the water weekly and dose a third of EI levels.
Everything in my tank is now inert after removing some seiryu stone and changing for a lump of lava rock. At the start of the week right after the WC before any dosing my TDS is around 127 (This is lowering weekly due to the rainwater water changes but measured from start of week then the end) The Difference between the start and the end is consistently 17 ppm, I add 27ppm of ferts through the course of the week which means that the net result is 10ppm of "something" but topping off would only account for a 0.24 change? Meaning the rest is a mixture of bio and food waste and whatever ferts are unused?
 

dw1305

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Hi all,
I seem to be getting some consistent results using a rule of third with this tank and good plant growth.
Perfect, just concentrate on that and forget all the numbers. At some point the TDS (conductivity) value will stabilize and you will get a datum range where you are happy with plant growth and fish health and the volume of water you need to change.
Am I right in thinking that if I have a 50 ltr tank with a TDS of 127 which loses about 4 ltrs of water per week and I top this water off with rain water with a TDS of 12 that I've raised the TDS by 12 divided by the 50 ltrs 0.24?
Sort of, you still have the same amount of TDS in the 46 litres that you had in the 50 litres, if that makes sense? What has evaporated is pure H2O, concentrating the salt solution that is left in the "water".

So you have 46 litres with 127ppm TDS and 4 litres with 12 ppm TDS which gives you 50 litres with a new mean TDS value of ((46*127) + (4*12))/50) = 118ppm TDS
The Difference between the start and the end is consistently 17 ppm, I add 27ppm of ferts through the course of the week which means that the net result is 10ppm of "something"
The best we can do is "something". Unfortunately the "ppm" of the salts you've added as fertiliser isn't the same as the "ppm" of TDS that you've measured via the conductivity meter, because differing ions conduct differing amounts of electricity.

cheers Darrel
 

AverageWhiteBloke

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So you have 46 litres with 127ppm TDS and 4 litres with 12 ppm TDS which gives you 50 litres with a new mean TDS value of ((46*127) + (4*12))/50) = 118ppm TDS
Yeah that bit was the one giving me the bad head. I was thinking it won't be 127 because it was 127 when there was 50 ltrs of water so now 4 ltrs of that has gone and left the salts so that would leave a higher TDS plus what I add in the top offs. :banghead: Maybe not haha.

Yeah for sure I'm trying not to get overly bogged down with the numbers, I try and keep life as simple as I can and spend more time enjoying the hobby. Reason I query it was since getting some Duck Weed and converting to rain I am aware that my TDS is dropping substantially. The tank when running on Tap water started off at a TDS of 265 and now it starts its week at 127 after water change.

I was also combining the TDS monitoring with some end of week nitrate testing (there I've said it :D) which were getting increasingly redder not that I give it much thought but it was worth pointing out. I did wonder if some ammonia might be getting in through the rain water with debris then converting in the tank 🤔 My biggest concern was for the RCS in there which tend to like a slightly higher TDS to molt properly (so I hear) Was wondering whether it might be worth my while swapping out some of the KNO3 for more KHCO3. I already dose 3ppm of MG and KHCO3 weekly as part of my dosing. Probably worth just adding more of this on top of what I'm already dosing I would say or put the seiryu stone back in. I just took it out to take it out of the equation which by my guestimates added about 10ppm weekly for the amount I had in there.

Anyway since changing to this regime plants have came on nicely, the Duck weed pointed out some serious shortfalls in dosing I reckon. I think I was seriously underestimating how much I needed to dose, most plants were failing but since upping to this third EI dosing the crypt balansae and amazon sword have boomed as has the the other cyptt wendt you can't really see at the back.

IMG_20201221_1215538.jpg


My fighter there wondering what all the fuss is about a nettle leaf :D

IMG_20201221_1218114.jpg


IMG_20201221_1216533.jpg


Definitely a huge improvement on what it was though after 6 weeks of dissolving plants, the Beuce is responding nicely. I just need my hair grass and siamensis B to make a come back so I can hopefully get them replanted again. I have what's left of the growing tips and some hair grass currently sitting in ICU getting some "aireal advantage" in some old Tropica soil I had left from a previous high tech life.


IMG_20201221_1219175.jpg


Playing the long game.
 

dw1305

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Hi all,
The tank when running on Tap water started off at a TDS of 265 and now it starts its week at 127 after water change.
Plant growth looks pretty good, I'd try and keep it about there.

I can use our consistently hard tap water if I want some more dKH, but I don't see any problem with using potassium bicarbonate (KHCO3) instead if your tap water <"is still quite variable?">
with some end of week nitrate testing (there I've said it :D) which were getting increasingly redder not that I give it much thought but it was worth pointing out.
My guess would be that it isn't measuring just the NO3- ion. You could try reducing the amount of potassium nitrate (KNO3)? but to be honest I'd just keep on going how you are.
I did wonder if some ammonia might be getting in through the rain water with debris then converting in the tank
Probably not, because you only have 12 ppm TDS, that is pretty close to distilled water and the TDS would include ammonium ions (NH4+) if the pH was less than pH7, but not NH3 gas if the pH was higher.

cheers Darrel
 

dw1305

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Hi all,
So you have 46 litres with 127ppm TDS and 4 litres with 12 ppm TDS which gives you 50 litres with a new mean TDS value of ((46*127) + (4*12))/50) = 118ppm TDS
That is still not right, despite saying the salts are concentrated in the 46 litres, I've still used the original 127ppm TDS value from 50 litres.

It should be 46 litres with 138 ppm (TDS 127/0.92 = 138ppm (0.92 is the fraction 46/50)) and a new mean TDS value of ((46*138) + (4*12))/50) =128ppm .

cheers Darrel
 

AverageWhiteBloke

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I can use our consistently hard tap water if I want some more dKH, but I don't see any problem with using potassium bicarbonate (KHCO3) instead if your tap
Yeah mate I guess I could just cut a bit of extra tap in there if needs be, it's currently coming out around 70 although this is up and down like a yoyo. We're currently in the middle of a major line switch between reservoirs and a lot of work going on on the line which is why I avoid it. They're currently "blending" from our original source and some underground aquifers only taking from the underground depending on demand. I usually put a kettle full or 2 ltrs into my 15 ltr WC mix so I'm not purely running on rain water which is why I was adding a touch of KHCO3.
Probably not, because you only have 12 ppm TDS, that is pretty close to distilled water and the TDS would include ammonium ions (NH4+) if the pH was less than pH7, but not NH3 gas if the pH was higher.
Yeah I have about a 50ltr butt. Soon as it's nearly full I take a 25ltr container from which gets me two weeks worth of water changes. I bin the rest in the bottom where all the moss and debris then rinse and repeat. Just sometimes I've found dead worms etc in there so thought they might have been decomposing a little.

You're right though now I read back. It's not broken so nothing to fix at the Mo. Thanks for your time Darrel while I think out loud ;)
 

AverageWhiteBloke

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It should be 46 litres with 138 ppm (TDS 127/0.92 = 138ppm (0.92 is the fraction 46/50)) and a new mean TDS value of ((46*138) + (4*12))/50) =128ppm
Which would account for the extra 10ppm I am finding? Hmm so maybe I am using all the ferts!
 

AverageWhiteBloke

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To be fair when it comes to rain water it says TDS 12 but it's a very cheap meter I can't calibrate. usually I find when I put it in the water it drops over time but in the rain water it just says 12 end of. It's good enough for what I'm trying to achieve though. When I add my first dose after WC it roughly measures correctly that first dose.
 

dw1305

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H all,
Which would account for the extra 10ppm I am finding? Hmm so maybe I am using all the ferts!
Your one third water change (~17 litres), with 12 ppm TDS rainwater, and a starting tank TDS value 138ppm TDS, would take the conductivity down to 96 ppm TDS.
usually I find when I put it in the water it drops over time but in the rain water it just says 12 end of.....but it's a very cheap meter I can't calibrate
My guess would be that it just means "less than 12 ppm TDS", because it has reached its lower detection limit.
You're right though now I read back. It's not broken so nothing to fix at the Mo.
The changes in conductivity that you are recording are tiny amounts of ions if you look at how <"they relate to weights of salts">.

cheers Darrel
 

AverageWhiteBloke

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Sounds about right mate, my last three weeks stats go along the lines of...

test.jpg


The changes in conductivity that you are recording are tiny amounts of ions if you look at how <"they relate to weights of salts">.

Yeah probably getting bogged down in detail mate. Plants are growing I just like correlating the figures to see roughly where I need to be with this tank. OFC as plants grow in more all this will change again :)
 

AverageWhiteBloke

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There's been a huge change in the tank already, I found myself getting a bit of shopping a couple of doors down from P@H and this post had my juices flowing. Decided to treat myself to two portions of S. Repens and a Portion of Bacopa compact. Ripped out what was left of the hair grass which was an algae magnet anyway and didn't seem to want to take root in my cat litter and planted out the foreground. I've left a few of the more healthy ones, you never know.

Gets expensive posting in here :D
 

sparkyweasel

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Hi all,

That is still not right, despite saying the salts are concentrated in the 46 litres, I've still used the original 127ppm TDS value from 50 litres.

It should be 46 litres with 138 ppm (TDS 127/0.92 = 138ppm (0.92 is the fraction 46/50)) and a new mean TDS value of ((46*138) + (4*12))/50) =128ppm .

cheers Darrel
I think that when @AverageWhiteBloke tops up it returns the volume to 50l, and would return the TDS to 127 if the top-up water was 0TDS, so it will actually be; 127 + the TDS in the top-up water, which is 12 (TDS) x 4 (litres added) / 50 (total litres), ie 0.96, call it 1 and we get the same result but I think this way is simpler. Maybe. :)
 

AverageWhiteBloke

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Bizarrely right at the very start I had the 12/50 ltrs 0.24 which if I had just x 4 would have also gave the same result! Either way we do it I think we can all agree that the amount of tds raise caused by topping off with rain water is the net total of f all. So topping off explains none of the excess at the end of the week which we've also established is f all either. Interesting little experiment though .... For me anyway. :)
 

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