This test kit includes everything you need to measure the general and carbonate hardness of your aquarium water. Benefits: Monitors GH & KH to create a more natural habitat Use in fresh water Follow the API Easy Care Guide and use GH/KH TEST KIT when: Testing key water parameters Identifying problems in an 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). hen 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 habitat of their fish, as well as detect low KH levels, which can lead to rapid and wide pH shifts, stressing fish.