API General/Carbonate Hardness KitTests tap water and freshwater aquariums for general hardness (GH), the measure of dissolved calcium and magnesium ion concentrations, and carbonate hardness (KH), the measure of dissolved carbonate and bicarbonate ion concentrations. Helps in detecting low KH levels, which can stress the fish by causing rapid pH shifts, and helps owners replicate the natural habitat of their fish. For freshwater use only. Simple 2 bottle test.