Aquarium Oxygen Test Kits
Oxygen test kits tell you how much dissolved oxygen there is in your aquarium water. Fishes need oxygen to live, and any aquarium needs proper aeration. Large tanks can suffer "dead zones" - areas that aren't getting oxygen - without adequate water circulation. Water pumps provide the most circulation; in a large tank you may need an air pump too, or an aerating water pump. Smaller tanks can often get by on an air pump alone.
Use oxygen test kits weekly if you have a large bio-load or a large tank, or to make sure your aeration setup is adequate.