Does measuring TDS give more accurate water hardness range? - The Planted Tank Forum
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post #1 of 3 (permalink) Old 03-25-2018, 12:35 AM Thread Starter
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Does measuring TDS give more accurate water hardness range?

Would testing the TDS of the water give you a more accurate picture of how hard your water is compared to the test kit with the drops?

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post #2 of 3 (permalink) Old 03-25-2018, 01:12 AM
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I can't imagine it would be less accurate. However, I remember reading somewhere that they found that a TDS meter can stress shrimp out.
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post #3 of 3 (permalink) Old 03-26-2018, 04:12 AM
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The short answer is: no.

The long answer is: there is a positive correlation between GH and TDS, but you can't separate the conductivity of everything else from Ca and Mg to see how hard is your sample by measuring the TDS.

TDS is basically the electrical conductivity of the water. Every ion has a contribution to this conductivity, some contribute more, other less:

Relative conductivity
H+ 7.0 OH– 4.0
Li+ 0.8 Cl– 1.5
Na+ 1.0 Br– 1.6
K+ 1.5 I– 1.5
Mg++ 2.1 NO3– 1.4
Ca++ 2.4 acetate 0.8
Zn++ 2.1 SO4— 3.2

So, by measuring the TDS you get the conductivity of basically everything that has a charge dissolved into the water. If you have two samples with the same GH but one has less Na+ and nitrates, other with more Na+ and nitrates, the TDS will show different results despite having the same GH.
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