Calcium, magnesium and GH question - The Planted Tank Forum
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post #1 of 6 (permalink) Old 06-26-2020, 09:58 PM Thread Starter
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Ive been trying to find my magnesium levels in my planted tank by subtracting GH minus calcium. I tested my GH which is at 5, calcium is reading at 20ppm or below using the nutrafin calcium test kit. I used a converter to change 5 GH to ppm and it comes to 83ppm, so 83ppm - 20ppm calcium would be 63 ppm magnesium, at first I thought this was right but it doesn’t make sense.

I used the Rotala butterfly calculator, to measure 8ppm of magnesium and 20ppm of calcium in one gallon, the 20ppm of calcium in one gallon brings the GH to 2.8, and the 8 ppm of magnesium changes it to 1.8, so together in one gallon is 4.6 gh, this is by having 20ppm of calcium and 8 ppm of magnesium, total of 28ppm in 4.6 GH. So why when I used a calculator online to turn 4.6 gh to ppm it says 76 ppm? This is very confusing to me.
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post #2 of 6 (permalink) Old 06-26-2020, 10:11 PM
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How do I test for Mg?
First test for GH and Ca, then calculate Mg.
Mg testing
((17.86 x dGH) - (2.5 x Ca ppm)) / 4.1 = Mg ppm
(ppm dGH - (2.5 x Ca ppm)) /4.1 = Mg ppm

((17.86 x 5dGH) - (2.5 x 20)) / 4.1 = Mg ppm
89.3 - 50 = 39.3 / 4.1 = 9.59 ppm Mg
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post #3 of 6 (permalink) Old 06-26-2020, 10:27 PM Thread Starter
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Quote:
Originally Posted by Immortal1 View Post
How do I test for Mg?
First test for GH and Ca, then calculate Mg.
Mg testing
((17.86 x dGH) - (2.5 x Ca ppm)) / 4.1 = Mg ppm
(ppm dGH - (2.5 x Ca ppm)) /4.1 = Mg ppm

((17.86 x 5dGH) - (2.5 x 20)) / 4.1 = Mg ppm
89.3 - 50 = 39.3 / 4.1 = 9.59 ppm Mg
Ok this makes way more sense thanks. I have another question though, which I know will sound stupid but I prefer that than to mess up, where does the 17.86 and 2.5 come from? I’m assuming I will always have to multiply my GH by 17.86 and my calcium by 2.5, but where are these two numbers from?
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post #4 of 6 (permalink) Old 06-27-2020, 01:55 AM
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PPS Pro web site
LOT of good info on this site
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post #5 of 6 (permalink) Old 06-27-2020, 02:10 AM Thread Starter
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Damn that’s a great site thanks &#x1f44d;
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post #6 of 6 (permalink) Old 06-27-2020, 02:25 AM
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More great info
https://www.advancedplantedtank.com/


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