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.