API 5-in-1 TEST STRIPS for freshwater and saltwater aquarium help fish owners measure levels of Nitrite, Nitrate, Carbonate & General Hardness, and pH found in aquarium water. In order for fish to thrive, water must be similar to that which could be found in their natural environment. Poor water quality can cause fish stress, illness and even death. Just dip a strip directly into your aquarium and use the colour-coded chart to determine if your water is safe, or if adjustments are needed.