TDS- the most confusing parameter??? - The Planted Tank Forum
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post #1 of 18 (permalink) Old 07-01-2011, 02:29 PM Thread Starter
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TDS- the most confusing parameter???

I am posting this to share my experiences and also hopefully answer some questions about this all encompassing parameter.

I purchased a TDS meter (Hanna Primo) about 3 years ago when I decided to undertake the task of breeding L-183 Starlight plecos. I knew I needed soft water to get them to breed so I figured it would help. Now after three years and keeping TDS ~100ppm they breed pretty prolifically.

I used the meter on my high tech 37g because I have soft water species in there (L.Pantanal, various eriocaulons, tonninas, syngonanthus, etc.). When I first began testing on the tank my TDS were in the low 100's. Then I had some deficiencies appear and started dosing GH booster and Epsom Salt. Over the course of following the correct dosing amounts for NPK, GH booster and Epsom the TDS rose to almost 700ppm! I know its due to the ferts and salt but WOW that was a shock.

All of the plants are doing wonderful so I have no specific issue I'm just trying to figure out the best possible way to determine actual water hardness in the tank. I hate liquid kits and I do not have a high end liquid kit for hardness. I would also assume that since the TDS is effected by dosing the GH would also be affected and perhaps also the KH because of the salt???


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post #2 of 18 (permalink) Old 07-01-2011, 03:11 PM
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I would also assume that since the TDS is effected by dosing the GH would also be affected and perhaps also the KH because of the salt???
Yep, if you dose GH booster, the GH will go up. KH not so much, it reflects mostly carbonate hardness, shouldn't be affected by adding GH booster.

TDS measures all ions dissolved in water, so it includes also things like sodium that don't contribute to water hardness.

The whole hardness/alkalinity thing is pretty confusing since some terms are used interchangeably. Add in TDS, EC, buffering capacity, and perhaps some phosphates, and it makes your head spin.


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post #3 of 18 (permalink) Old 07-01-2011, 05:24 PM Thread Starter
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It seems as if the Epsom salt really makes the the TDS skyrocket.

So what exact measurements would one need to determine the hardness of the tank water without being thrown off by the ferts we add.


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post #4 of 18 (permalink) Old 07-01-2011, 05:40 PM
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So what exact measurements would one need to determine the hardness of the tank water without being thrown off by the ferts we add.
Water hardness, much like TDS, is produced by a variety of ions. In the case of TDS, all ions increase it. In hardness, calcium and magnesium are the main contributors, though iron and manganese are also counted. So it is directly changed by adding fertilizers.

The "gold standard" for measuring hardness the most accurately would be to measure the concentration of each ion, convert to CaCO3 equivalent, and add them up - you could use an ICP-OES instrument for this (fastest - it measures all the ions at once - but costs about $125,000).

If you just use a liquid hardness test kit you get results very close to those of the expensive instrument. There are also calcium ion-selective electrodes (and maybe Mg, not sure) that could help - but the liquid test is usually better.

If you want a really cheap way (but less accurate) - use a dilute soap solution. Count how many drops you need to add to 10ml of water to get soap bubbles after shaking. Initially it will form soap scum, so no bubbles. You'd also have to calibrate this with a known hardness solution.

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post #5 of 18 (permalink) Old 07-01-2011, 08:14 PM Thread Starter
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Thanks for the info Kevin.

So what OTC test kits would someone need to calculate hardness without having skewed results from dosing, or is that even possible?


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post #6 of 18 (permalink) Old 07-01-2011, 09:14 PM
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So what exact measurements would one need to determine the hardness of the tank water without being thrown off by the ferts we add.
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So what OTC test kits would someone need to calculate hardness without having skewed results from dosing, or is that even possible?
Define "hardness"...

You could measure the tap (or RO/DI) water you use for water changes. That would be your starting point without added fertilizers.

The answer to your question is most likely "not possible". Epsom salt is Magnesium Sulfate, and when you measure hardness, you measure Magnesium (among other things), so that's really a part of the measurement, not "throwing off" or "skewing" it. If you need soft water don't add GH booster...


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Last edited by Wasserpest; 07-02-2011 at 12:40 AM.
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post #7 of 18 (permalink) Old 07-01-2011, 10:17 PM
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Don't dose the water column. Keep all the nutrients in the substrate and plant heavy root feeders.
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post #8 of 18 (permalink) Old 07-01-2011, 10:38 PM
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Define "hardness"...

You could measure the tap (or RO/DI) water you use for water changes. That would be your starting point without added fertilizers.

The answer to your question is most likely "not possible". Epsom salt is Calcium Sulfate, and when you measure hardness, you measure Calcium (among other things), so that's really a part of the measurement, not "throwing off" or "skewing" it. If you need soft water don't add GH booster...
Epsom salt is Magnesium sulfate.

Just use the hardness kit. I suppose one way would be to precipitate out all the minerals in a set volume of water and take the mass. But this would mean some serious number crunching and a scale that would cost like 12 grand. As far as we're concerned test kits are plenty accurate. Just simple titrations are good enough for us.

And at the above person. That' easier said than done. And not very relevant to the discussion...

TDS is one of those pointless measurements IMO. Sure you don't want like 10000 TDS. But in the wild TDS are much much higher than what we have in our tanks with their fancy mechanical filtration. Many soft water fish are found in waters with TDS as high as 1200. Don't bother with it...

One use for TDS meter: Checking if your RO/DI system is still good. That's all.
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post #9 of 18 (permalink) Old 07-01-2011, 11:10 PM
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I thought that the soft water one spoke of when refering to blackwater and other soft water species, was the KH, carbonate hardness, and not so much GH. I don't know for sure.
API makes a very affordable KH test, I picked one up for $7 dollars.
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post #10 of 18 (permalink) Old 07-02-2011, 12:46 AM
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Epsom salt is Magnesium sulfate.
Whoops you are right. Been a long day.

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And at the above person. That' easier said than done. And not very relevant to the discussion...
Why not? B is wondering how to maintain soft water and still grow plants without deficiencies...

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TDS is one of those pointless measurements IMO. Sure you don't want like 10000 TDS. But in the wild TDS are much much higher than what we have in our tanks with their fancy mechanical filtration. Many soft water fish are found in waters with TDS as high as 1200. Don't bother with it...

One use for TDS meter: Checking if your RO/DI system is still good. That's all.
Interesting info. I thought typical softwater fish are found in waters with very low TDS. Are you saying fancy mechanical filters filter out those dissolved solids? Resulting in lower TDS figures?


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post #11 of 18 (permalink) Old 07-02-2011, 01:10 AM
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I used to track gh/kh of my tanks. The plants have all been doing just fine so I started thinking I was wasting my time testing and stopped worrying about it as long as they were healthy. Haven't looked back since.

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post #12 of 18 (permalink) Old 07-02-2011, 01:36 AM Thread Starter
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I use RO water so I must add some sort of GH booster. It just seems that with the test kits that are available at a reasonable price there is no way to accurately figure out exactly what the water hardness in the tank is because the ferts we put in screw up the readings.


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post #13 of 18 (permalink) Old 07-02-2011, 01:45 AM Thread Starter
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So I posted these pics in the wrong thread.



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Last edited by bsmith; 07-02-2011 at 03:19 PM.
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post #14 of 18 (permalink) Old 07-02-2011, 04:52 AM
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Whoops you are right. Been a long day.



Why not? B is wondering how to maintain soft water and still grow plants without deficiencies...



Interesting info. I thought typical softwater fish are found in waters with very low TDS. Are you saying fancy mechanical filters filter out those dissolved solids? Resulting in lower TDS figures?

TDS is a measurement of not only dissolved minerals but also dissolved organic compounds. Imagine those muddy black waters most fish come from. The TDS in those waters is sky high. Or filters get rid of the majority of organic solids be they molecular (chemical filtration) or macroscopic (mechanical filtration).
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post #15 of 18 (permalink) Old 07-05-2011, 02:55 PM
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So what exact measurements would one need to determine the hardness of the tank water without being thrown off by the ferts we add.
The only way would be with gH and kH test kits.

Your TDS is also going way up because of all the potassium contained in gH boosters.
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