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
You have Hagen calcium kit and it gives number in Ca, not CaCO3, so Mg would be:
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)
I am not looking to get a certain number on this mineral butt i am looking for a good range to be in. I have a high light tank so from i am told they nutrient suckers big time. Niko as far as the 5 ppm that u gave me that is close to the number i had (4.87). Aloso niko i want to thank you for calculator i will use in the future. Shalu i do hagen test kits both measure in 20 ppm intervals i get what u mean that have certain range to them and that they dont show dead on what the tank is. This what i use to figure out the Mg.
(gh in ppm) - (2.5 x ca ppm)