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Guide to TDS

DaveWatkin

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26 Oct 2020
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Aberdeen, UK
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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25 Sep 2014
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Torbay
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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