API Test GH & KH Hardness KitThe API GH & KH Hardness Test Kit includes everything you need to measure the general and carbonate hardness of your aquarium water. Water hardness is a measure of the dissolved mineral content of water. There are two types: the general hardness (GH) and carbonate hardness (KH). When keeping freshwater fish and plants, the GH and the KH of aquarium water should match their natural habitat. The GH & KH TEST KIT helps aquarists duplicate the natural