his API GH/KH Test Kit includes everything you need to measure the general and carbonate hardness of your freshwater aquarium. 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 API GH/KH Test Kit helps aquarists duplicate the natural habitat of their fish, as well as detect low KH levels which can lead to rapid and wide pH shifts stressing fish.