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Guide to TDS

DaveWatkin

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26 Oct 2020
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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
 

Nick potts

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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
 

dw1305

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Hi all,
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.
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.
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.
......... 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".

If the amount of sugar in both pans is balanced, the pH is pH7. You can have a grain of sugar in either pan, a sugar cube in either pan or a 2 kg bag in either pan and the pH is still pH7, as long as we are in balance.

If you add a grain of sugar to the "acid" pan that already contains a grain of sugar, you now have twice the mass of "acids" compared to "bases" and the pH falls to a fairly low level, but we've only added one grain of sugar. If we then repeat the exercise with the balanced 2 kg bags, the added grain of sugar to the acids side now makes no practical difference to the balance and the pH remains at pH7.

If you now think of the "acid" grain of sugar as a "H+ ion donor" and the "basic" grain of sugar as a "H+ ion acceptor", we are pretty close to understanding pH........
In very soft water <"pH can never be stable, and this doesn't matter">.
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.

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This is the bit where probability comes into play, after making certain assumptions.
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
1611582043612.png


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 😬
 

DTM61

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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.
 

Zeus.

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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
 

dw1305

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Hi all,
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).
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

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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?
 

arcturus

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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.
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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hypnogogia

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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?
 

LondonDragon

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Perhaps UKAPS would benefit from a glossary or similar? ;)


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

dw1305

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Hi all,
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
 

AlecF

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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.
 

arcturus

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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.
 

AlecF

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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.
 

arcturus

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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.

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.

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

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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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