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Use this:

http://www.deepforestaquatics.com/aqua/CaMgRatio.html

and enter GH=4 and Ca=20. Then enter GH=4 and Ca=30. The Mg becomes negative with only 10 ppm difference.

If I read the "FAQ" right (http://www.aquaticplantcentral.com/forumapc/showthread.php?t=2427) then if the water has only Ca then 1 dGH is provided by 7 ppm Ca. So in the example above with GH=4 if the Mg=0 then the Ca cannot be more than 7 x 4 = 28 ppm. But it is not unusual to get test results of say Ca=40 and GH=4 maybe because the resolution of the test kit I use is 10 ppm.

I don't know if the formula is not entirely correct or what. We left it as it is, showing negative numbers, because it seemed to give you an idea how far is the spread between Ca and Mg when the Ca was way more than the Mg.

To me personally the bottom line is what Edward has been preaching lately - one must have Ca and Mg in a ratio that is close enough. Ratious like 4:1 or 10:1 are not written in stone but a 100:1 ratio is most likely way off. And surely one should not have a negative number

--Nikolay

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Mg = (220 - 80*2.5)/4.1 = 5ppm

But Ca has a resolution of 10 ppm, I guess GH the same resolution?

The Mg calculation can be up to 6 ppm off, I would say it is between 2.5-11 ppm( assuming 220 GH really means 210-220ppm, 80ppm Ca really means 70-80ppm)

The formula that I referred you to doesn't use ppm but dGH. 1dGH is 17.9 ppm, so:

220/17.9 = 12.3 dGH

Then if you use GH=12 dGH and Ca=80 ppm you get:

Mg=3.5 ppm

Ca:Mg = 23:1

I agree with shalu, the tests are not very precise and that is why I believe that some Mg and some Ca should be present but there is no need to chase precise ratios.

--Nikolay

(gh in ppm) - (2.5 x ca ppm)

________________________________

4.1

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What if one uses 20ml, 50ml? Would it still work, just uses more reagent?Edward said:Hi BigFoot

If you use 10ml instead of the 5 ml then your Ca resolution is 10 ppm, not 20 ppm. With Hagen Ca test kit.

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Hi Shalushalu said:What if one uses 20ml, 50ml? Would it still work, just uses more reagent?

Yes, we can use more water. With the Hagen Ca test kit we still want to put 7 drops of #1 and 3 drops of #2.

Solution #3:

1 drop in 5 ml = 20 ppm

1 drop in 10 ml = 10 ppm

1 drop in 20 ml = 5 ppm

… and so on …

The limit is the visibility of the color change from pink to blue.

Edward

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