Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
The GH (General Hardness) tests Calcium and Magnesium. KH (Carbonate Hardness) tests carbonates and bicarbonates.
In various sorts of limestone, calcite, dolomite and related minerals there are different amounts of calcium carbonate and magnesium carbonate.
When water passed through these minerals (especially slightly acidic rain water which has picked up some CO2) these minerals dissolve in the water.
This is what makes tap water (or the water in the Rift Lakes and other places) hard.
Occasionally other minerals may be found in such water, including potassium. But potassium is not part of the GH test.
I am not sure why they add potassium sulfate to GH boosters. If you do not want to add potassium sulfate you can get the other ingredients (calcium sulfate and magnesium sulfate are the most common) or substitutes such as calcium chloride and raise the GH of the water with the right ratio of whatever you select.
Some tap water does not have both Ca and Mg, or they are both there, but in the wrong ratio. You can supplement whichever is needed.
Plants use Ca and Mg in a ratio of about 4:1. The water does not have to have this exact ratio, but something close is good.