Prevent excess phosphate in your fresh or saltwater tank with API Phosphate PO4 Fresh & Salt Water Aquarium Test Kit. Phosphate can come from tap water, waste, uneaten food or dead algae. In freshwater tanks, high phosphate levels can cause algae and green water blooms; in saltwater tanks, it can stunt the growth of hard corals. This easy-to-use test kit measures phosphate levels from 0 to 10. Simply fill a test tube with your tank water, add a few drops of both test samples and shake. Then compare your results to the color card and follow instructions to lower phosphate if necessary. Frequent testing is recommended to help maintain healthy aquarium conditions.