In order for fish to thrive, water conditions in the aquarium need to be the same or very similar to those found in nature. The API 5 in 1 Aquarium Test Strips measures 5 of the most important aquariums conditions: pH, Nitrite, Nitrate, Carbonate and General Hardness. Improper levels can cause fish stress, which can lead to illness and death. This kit contains 4 test strips, easy to read instructions, and information on how to correct unsafe water conditions. For freshwater and saltwater aquariums. Test weekly to make sure water parameters do not reach undesirable levels.