The API GH and KH test kit tests the levels of carbonate hardness and general hardness in the aquarium. KH or carbonate hardness in high enough levels prevents pH from changing rapidly and maintains a healthy system. GH or general hardness is a measurement of how many dissolved minerals are present in your water. Depending on the fish being kept would determine how much hardness should be present in the water. Guppies, Mollies, Platies, Swordtails, Goldfish, African Cichlids and more like high mineral content as it matches their natural habitat. Ensure proper levels of both KH and GH in your aquarium with this test kit.