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dennis said:
Well I was finally to check my waer chemistry today. TDS from Hanna meter and Ca levels from Salifert test

Tank: Tap:
TDS 264 ppm TDS 96ppm
GH 12*= 213.6ppm GH 8*=142.4
Ca 150ppm Ca 120ppm
Mg 63ppm Mg 22ppm

That means that my tapwater has pretty good parameters but my tank water is out of wack due to, I assume, my heavy dosing of CSM and the use of and Onyx/Eco mixed substrate.

Also, can the tap measurments be correct? Is it possible to have a Gh higher than your TDS?
Your Ca and Mg numbers are stilll in CaCO3 equivalent numbers, the real ppm for Ca and Mg are:

Ca 60ppm 48ppm (divide your numbers by 2.5)
Mg 15ppm 5 ppm (divide your numbers by 4.1)

I guess it is possible for TDS and GH to mismatch to some degree. There are assumptions built into each that neither truely reflects the real ppm in your water. TDS measures electrical charges and assume that they are from a given composition of ions(depending on factory calibration fluid used, for example, NaCl, KCl, or some other salt mixture) and convert it into ppm. On the other hand. GH measures the total multivalent ions(mostly Ca++ and Mg++ typically) and assume that they are all from CaCO3 and converts it into CaCO3 ppm in most test kits. Still, it is a bit odd your tap TDS is so much lower. I did a simple calculation, assuming you only have GH ions in water and your TDS uses NaCl, it should actually give slightly higher reading than GH. Due to the electrical charge differences, TDS meter should see two NaCl molecules(molecular weight 56) for every CaCO3(molecular weight 100) molecule seen by the GH kit, unless this assumption of mine is wrong and they see it one for one.
 

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Keep in mind that our tests have limited resolution and your tap water only has 5ppm Mg, so it could have been less. I bet the problem was TOO LITTLE Mg in the beginning. Now that you have about 15ppm Mg, things look better. I had similar stunting issues, adding Mg via epsom salt helped a lot.
 

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I know people say Mg affects mostly old growth, but my problem was mainly with new growth, and adding epsom salt definately helped a lot, especially with alternanthera reneckii stunting problem. I think mg in CSM will hardly make any dent in terms of ppm, did you estimate how much it was really adding in terms of ppm in the tank? Ok, I did a rough estimate, typically it will be 1ppm per week of dosage, 2ppm Mg per week from very heavy dose of CSM.
 
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