# Guide to TDS



## DaveWatkin (25 Jan 2021)

I see a lot of recommendations for using TDS in water prep but I don't know much about it. Does anyone have a good reference for reading up? Looking at you Darrel, you always seem to come up with excellent advice fast 

Main things I don't understand are how to use TDS as a replacement for both Gh and Kh tests? My tapwater has 2Gh 0 Kh most the time so I have a large water barrel and add seachem products to get a nice 6/6 mix which I use in all my tanks. I only realised I had issues with tapwater when I was having big Ph swings in my first tank and starting using products based on advice from LFS so Kh is my biggest concern as I want to avoid the big swings.

Thanks


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## Nick potts (25 Jan 2021)

TDS is basically just a measure of dissolved minerals, salts etc in the water.

When people talk about using a TDS meter to judge GH it is usually when using RO water and a remineralizer with a known composition, I am not sure using tap water will work as the TDS meter doesn't know what's in the tap water, just the conductivity which could be anything.

So in my case, I know if I mix my 0TDS RO water with JBL aquadur to a TDS of 180 it gives my roughly 6GH and 5KH water ( i think  )

Reading that back I suck at trying to explain things, and I am sure someone will be along to explain better shortly, @dw1305 lol


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## dw1305 (25 Jan 2021)

Hi all,


DaveWatkin said:


> Does anyone have a good reference for reading up?


I'll try and link in threads <"that cover all the moving bits">, but it is <slightly problematic> because nearly all the units are derived, and in some cases you are reliant on probability, rather than an empirical value.

TDS is "Total Dissolved Solids" It is a measure of all the compounds that are dissolved in the water. What we call "water" isn't H2O, but a dilute solution of ions and non ionic compounds ("the solutes") with H2O as the solvent.  If you want to measure "ppm TDS" you need to filter (to get rid of "undissolved solids") and then <"evaporate a large volume of water to dryness"> and weigh the residue.
A TDS meter doesn't actually measure TDS, it measures Electrical Conductivity (EC) and then uses a <"conversion factor"> to estimate the ppm TDS from the amount of ions dissolved (non-ionic compounds don't conduct electricity).
Electrical <"conductivity has a linear"> relationship with ions in solution.
So that is the TDS bit.


DaveWatkin said:


> I don't understand are how to use TDS as a replacement for both Gh and Kh tests?


This is the bit where probability comes into play, after making certain assumptions.

@Nick potts  is right there is no inherent link between conductivity, dGH or dKH and no link between dGH and dKH either, but in *most freshwater* all three values are linked because <"they are all relate to the amount of dissolved limestone"> (CaCO3) in the water. <"In Torbay"> (where Nick resides) you can be 99.9% sure that the link for all three parameters is true.
Calcium carbonate <"isn't soluble in water">, but it is <"soluble in weak acids">, and rainwater is a weak acid because of the amount of dissolved CO2 it contains. At 400ppm CO2, the solubility of CaCO3 (in pure H2O), means that water from a limestone aquifer has about 18 dGH and 18 dKH. The derivation of both <"GH" and "KH" is a bit strange"> but fully described in <"Larry Frank's "the Krib" article">.



DaveWatkin said:


> I only realised I had issues with tapwater when I was having big Ph swings in my first tank and starting using products based on advice from LFS so Kh is my biggest concern as I want to avoid the big swings.


Just don't listen to them. I'll be charitable and say that they don't understand the science, rather than saying they do understand the science and took the chance <"to sell you a product">.  While conductivity is a really nice straight forward measurement, pH isn't, mainly because it is both a log10 scale <"and a ratio">.

When you have more than about 2 dKH your "resting" pH will go up to pH7.8, but it is easy to reduce because you have very few bases (H+ ion acceptors) in solution.


dw1305 said:


> ......... I think another problem is that people who have experience of pH in strongly buffered systems (like you might use for Rift Lake Cichlids) then extrapolate their (entirely valid) experiences to soft, low conductivity water. The problem is that in heavily buffered water falls in pH are caused by large changes in water chemistry, but as water approaches pure H2O pH becomes a less and less meaningful measurement.
> 
> Rather than thinking of pH as an absolute measurement, I find it easier to think of it as a ratio, and rather than thinking of rather abstract H+ and O-H ions, I think of pH as "grains of sugar" in a 2 pan balance, with one pan "acids" and the other "bases".
> 
> ...


In very soft water <"*pH can never be stable, and this doesn't matter**">. *


dw1305 said:


> What has really helped me is to think about changes in water chemistry, rather than just changes in pH.
> 
> In soft water small changes in water chemistry cause large changes in pH and
> in hard water large changes in water chemistry cause small changes in pH.



cheers Darrel


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## Zeus. (25 Jan 2021)

dw1305 said:


> This is the bit where probability comes into play, after making certain assumptions.
> 
> @Nick potts is right there is no inherent link between conductivity, dGH or dKH and no link between dGH and dKH either, but in *most freshwater* all three values are linked because <"they are all relate to the amount of dissolved limestone"> (CaCO3) in the water. <"In Torbay"> (where Nick resides) you can be 99.9% sure that the link for all three parameters is true.
> Calcium carbonate <"isn't soluble in water">, but it is <"soluble in weak acids">, and rainwater is a weak acid because of the amount of dissolved CO2 it contains. At 400ppm CO2, the solubility of CaCO3 (in pure H2O), means that water from a limestone aquifer has about 18 dGH and 18 dKH. The derivation of both <"GH" and "KH" is a bit strange"> but fully described in <"Larry Frank's "the Krib" article">.


CaCO3 is a tricky one esp like with what @Wookii experienced in his tank when he was using RO water CO2 injection and Seiryu stone the he was getting 5-6dGH and 5-6dKH increase alone from the rocks.

I am trying to incorporate a new feature into the IFC calculator called 'CaCO3 Max' which for a given pH will give you the increase in dGH/dKh for any pH without having the user interoperate values from one of the graphs below





Got it working fine with the various graphs and various bits of units conversions, but being able to enter any pH and get the CaCO3 g/l limit is the goal 😬


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## Zeus. (25 Jan 2021)

Did a little experiment with a 1.0M Citric Acid solution ( 4 teaspoons of citric acid in 100ml of warm preboiled tap water)


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## DTM61 (25 Jan 2021)

Zeus. said:


> CaCO3 is a tricky one esp like with what @Wookii experienced in his tank when he was using RO water CO2 injection and Seiryu stone the he was getting 5-6dGH and 5-6dKH increase alone from the rocks.


 Do you know how much stone he was using by any chance? I'm trying to harden my water but would like to know thoughts on how certain quantities of stone would balance this. I think it's more natural and am enjoying trying to figure it out.


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## Zeus. (25 Jan 2021)

I'm sure @Wookii would inform us, tried to find the thread he had or was catting about it but failed, I would not be keen to use CaCO3 based rocks again with a CO2 injected tank esp if using RO water. You can have the same effect by just dry dosing CaCO3 in excess and save your rocks


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## dw1305 (25 Jan 2021)

Hi all,


Zeus. said:


> Did a little experiment with a 1.0M Citric Acid solution


Pretty conclusive. Because it was citric acid (C₆H₈O₇), you've converted the CaCO3 to <"calcium citrate"> and CO2 (gas).


DTM61 said:


> I'm trying to harden my water but would like to know thoughts on how certain quantities of stone would balance this. I think it's more natural and am enjoying trying to figure it out.


You can just use a small amount of <"cockle/oyster shell chick-grit">, when it is <"fully dissolved just add a bit more">. I've got hard tap water, so I can use that, but I know people who have used <"shell grit successfully">.

"Hard water" (high dKH/dGH) just means that the carbonic acid (H2CO3) has already dissolved the CaCO3, where rain-water has encountered limestone.

cheers Darrel


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## Flukeworld (10 Dec 2021)

Sorry to highjack this thread, but I still don't get it.
I have tap which is GH3, KH2. I add some magnesium and calcium on water change to reach GH5 and have enough ppm of Magnesium and Calcium for the week, some baking soda to reach KH4 just because I see a lot about "its better" for plants, but also I have some livebearers which love it. What TDS will be for in this? Should I measure it at all, would it give me hints I do something wrong or have to do something more? 
Total dissolved Solids should raise a lot using the quantity of Magnesium Sulphate, Calcium Chloride and Sodium Bicarbonate I add. IF so, why measure TDS?


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## arcturus (25 Jan 2022)

Flukeworld said:


> Sorry to highjack this thread, but I still don't get it.
> I have tap which is GH3, KH2. I add some magnesium and calcium on water change to reach GH5 and have enough ppm of Magnesium and Calcium for the week, some baking soda to reach KH4 just because I see a lot about "its better" for plants, but also I have some livebearers which love it. What TDS will be for in this? Should I measure it at all, would it give me hints I do something wrong or have to do something more?
> Total dissolved Solids should raise a lot using the quantity of Magnesium Sulphate, Calcium Chloride and Sodium Bicarbonate I add. IF so, why measure TDS?


TDS supports several use cases. For example

Gauge the amount of compounds that are dissolved in the water. Example: you add x ppm of remineralizer salts and estimate the resulting TDS. With the TDS measurement you can double check if the amount of remineralization salts is in line with what you expect.
Check the dissolution of a salt. Example: you add x ppm of a salt and expect full dissolution in water. But TDS shows you that only a percentage of that salt is actually dissolved.
Check the variation (accumulation, removal) of compounds over time. Compounds can deplete or build up over time due to the addition of fertilizers, food, and to processes taking place inside the tank, such as plant intake, decomposition of matter, release of minerals from rocks (read the case above about the Seryiu stone), etc.. TDS will not tell you which compounds are changing but will tell if there is a variation of the total dissolved solids in the water column.
...
Note that a TDS probe is indirectly estimating the TDS value, not actually measuring it.


dw1305 said:


> A TDS meter doesn't actually measure TDS, it measures Electrical Conductivity (EC) and then uses a <"conversion factor"> to estimate...


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## jaypeecee (25 Jan 2022)

dw1305 said:


> A TDS meter doesn't actually measure TDS, it measures Electrical Conductivity (EC) and then uses a <"conversion factor"> to estimate...


Hi @dw1305

Perhaps UKAPS would benefit from a glossary or similar? 

JPC


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## dw1305 (25 Jan 2022)

Hi all,


jaypeecee said:


> Perhaps UKAPS would benefit from a glossary or similar?


I think it would. I have a feeling there is one somewhere Paulo (@LondonDragon) may know?

cheers Darrel


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## hypnogogia (25 Jan 2022)

dw1305 said:


> Hard water" (high dKH/dGH) just means that the carbonic acid (H2CO3) has already dissolved the CaCO3, where rain-water has encountered limestone.


Does this mean that when you leave hard water to degas so that all the CO2 is out, does the CaCO3 precipitate out?


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## LondonDragon (25 Jan 2022)

jaypeecee said:


> Perhaps UKAPS would benefit from a glossary or similar?








						Useful glossary of terms and acroynms
					

The planted tank subject can be somewhat daunting for the newcomer and less 'scientific'.  Here's a useful list of commonly used chemical names, acryonms and abbreviations etc.  Courtest of Tom Barr's - The Barr Report.  Those chemical names and acroynms! Add more if you see one I missed!!!   C...



					www.ukaps.org
				




If stuff is missing, you can always post on the thread to be updated!!


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## dw1305 (25 Jan 2022)

Hi all, 


hypnogogia said:


> Does this mean that when you leave hard water to degas so that all the CO2 is out, does the CaCO3 precipitate out?


It will only precipitate out if there was more CaCO3 in solution (as Ca++ and 2HCO3-) than there would be at equilibrium with  atmospheric CO2 levels. You often get this in <"tufa springs">  where the water is both <"cool and under pressure">.

cheers Darrel


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## AlecF (25 Jan 2022)

As a newbie I get confused trying to get my TDS down – was c 300, now c 250, or, after a big water change, as low as 200, but adding bicarb of soda or equilibrium to keep the KH up into the green will put it back up, as will APS ferts? I am doing c 10ml of micro and macro, alternate days, in a 100 litre tank. I have some cuttle fish in the tank. I keep meaning to do a thread on this, but here I am, butting in, lost and confused.


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## arcturus (25 Jan 2022)

AlecF said:


> As a newbie I get confused trying to get my TDS down – was c 300, now c 250, or, after a big water change, as low as 200, but adding bicarb of soda or equilibrium to keep the KH up into the green will put it back up, as will APS ferts? I am doing c 10ml of micro and macro, alternate days, in a 100 litre tank. I have some cuttle fish in the tank. I keep meaning to do a thread on this, but here I am, butting in, lost and confused.


Not sure I get your question.  dKH and dGH mineralizers increase TDS. Ferts also increase the TDS. Food, detritus, etc. will also increase it. You need a water change to remove the excess nutrients and dissolved compounds to reset the TDS.

And why do you want to decrease TDS in first place? If you want a lower TDS (or lower dGH or dKH) then you can use rain water or RO water. Otherwise, the TDS of your tap water will be your baseline.


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## AlecF (26 Jan 2022)

Thanks. The TDS on my tap water is much lower, of course, and I am aware what makes up TDS. I feel the fish are happier when it's lower than 200. I understand what is raising it. I feel the tank needs the mineralisers as my tests are lime green verging on yellow for carbonates and my rams horn and nerve snails were doing badly, which I thought related to that? I don't want to install RO as I share some people's doubts about the wastage. I collect a little rainwater in a bucket but can't afford a barrel at this time, and it's a communal garden. I live up 4 flights of stairs so rainwater isn't the easiest solution. I think I was overdosing ferts. I've been having some threads of algae. I have reduced lighting to 40% which has helped. And I reduced the ferts to 10ml. If anything I change water too much. I suppose my question is a bit dumb, but I would like to find a way to dose correctly and have a TDS closer to 150.  I'm aware I have a lot of learning to do and sometimes these kinds of issue seem unresolvable to a newbie whereas they are simple for someone experienced.


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## arcturus (26 Jan 2022)

AlecF said:


> Thanks. The TDS on my tap water is much lower, of course, and I am aware what makes up TDS. I feel the fish are happier when it's lower than 200.


Your minimum TDS depends on what is coming out of your tap. You would only get a TDS of 150 or lower if your tap water is on the "soft" range. But since it seems you are in Scotland, you might have indeed a very soft tap water due to the water sources in the region. Also note that the TDS is measuring all compounds dissolved in the water, not only the ones related to GH and KH.



AlecF said:


> I understand what is raising it. I feel the tank needs the mineralisers as my tests are lime green verging on yellow for carbonates and my rams horn and nerve snails were doing badly, which I thought related to that? I don't want to install RO as I share some people's doubts about the wastage. I collect a little rainwater in a bucket but can't afford a barrel at this time, and it's a communal garden. I live up 4 flights of stairs so rainwater isn't the easiest solution.


Snails need calcium. This implies a GH that is not too low, say above dGH 6 or 7. Note that GH measures calcium and magnesium, so a high GH does not necessarily means there is sufficient calcium in the water. Their shell will also erode if pH is too acidic (which can be an issue with CO2 injection). If your water is indeed soft and the carbonate tests are not looking good, then you should raise GH and add calcium. But this will increase the TDS - no way around this.



AlecF said:


> I think I was overdosing ferts. I've been having some threads of algae. I have reduced lighting to 40% which has helped. And I reduced the ferts to 10ml. If anything I change water too much. I suppose my question is a bit dumb, but I would like to find a way to dose correctly and have a TDS closer to 150.  I'm aware I have a lot of learning to do and sometimes these kinds of issue seem unresolvable to a newbie whereas they are simple for someone experienced.


Overdosing ferts will not cause algae. Otherwise all tanks that are using the Estimative Index fertilization regime would be a bowl of algae  Of course, the more ferts you add, the higher the TDS value. But your primary goal is to provide sufficient nutrients to the plants and not to let plants to starve because the TDS values are "high"! Algae are often caused by an imbalance in the system: too much light for the available nutrients, unstable or insufficient CO2 (in case you are using it), not enough plant mass, insufficient water circulation inside the tank, insufficient water flow, ...

In short: if you are not using water with low TDS (rain, RO), then it is not realistic to pursue a low TDS value after remineralizing the water and adding sufficient ferts. Your target should be to have all the minerals that your livestock needs and all the ferts that the plants need, regardless of the resulting TDS.

PS: a shrimp and snail breeder around here uses tap water in his tanks: dGH = ~15, dKH = ~10, TDS = ~320ppm (in the TDS 0.64 scale => EC = ~500 microSiemens) ...


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## AlecF (26 Jan 2022)

Thanks. TDS on tap water here is 55 ppm. And yes, it is soft water. It's reassuring, in a sense, that there's no way around the problem. I don't use CO2. PH is 6.5 or thereabouts. I will keep adding the bicarb of soda and the equilibrium. I have good circulation – a fluval 307 on a 100 litre tank. I will learn to accept a higher TDS and not worry so much.


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## dw1305 (26 Jan 2022)

Hi all, 


AlecF said:


> I have some cuttle fish in the tank.


That should be fine, you shouldn't need to add any more dKH. The rate of dissolution of the cuttle "bone" will depend on how soft the water is, softer water will dissolve the CaCO3 more quickly. 


AlecF said:


> I will keep adding the bicarb of soda


I'm not a fan of sodium (Na) addition, I don't think you need to add it, but if you did want an alternative potassium bicarbonate (KHCO3) would be better.

cheers Darrel


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## AlecF (26 Jan 2022)

dw1305 said:


> Hi all,
> 
> That should be fine, you shouldn't need to add any more dKH. The rate of dissolution of the cuttle "bone" will depend on how soft the water is, softer water will dissolve the CaCO3 more quickly.
> 
> ...


Thanks Darrel. I have ordered some – can you suggest a suitable quantity for a 100 litre tank. My usual carbonate test strip is the lightest green with a hint of yellow.


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## dw1305 (26 Jan 2022)

Hi all, 


AlecF said:


> can you suggest a suitable quantity for a 100 litre tank. My usual carbonate test strip is the lightest green with a hint of yellow.


I like a 100 litre tank,  it makes the calculations easier. For potassium bicarbonate (KHCO3) it is: 

7.2 g of KHCO3 in hundred  litres gives 2dKH and that should be plenty along with the cuttle bone.

Don't worry about the strips, just use the calculation, (from <"James' Planted Tank">). If you want the full workings they are in Larry Frank's article at <"the Krib">.


> 1.5g NaHCO3 in 25 litres of water = 2 dKH
> 1.8g KHCO3 in 25 litres of water = 2 dKH
> 1.2g K2CO3 in 25 litres of water = 2 dKH



cheers Darrel


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## jaypeecee (27 Jan 2022)

LondonDragon said:


> If stuff is missing, you can always post on the thread to be updated!!


Hi @LondonDragon 

Thanks for that. 

JPC


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