API pH Down adjuster lowers pH to make freshwater aquariums more acidic for fish and plants when levels rise too high. pH is the measure of acidic and alkaline conditions in water, where a value of 7.0 corresponds to a neutral pH, pH values above 7.0 are considered to be alkaline, and pH values below 7.0 are considered to be acidic. Different fish need different pH levels to be healthy, so it is important to test your water regularly to ensure pH levels are where they should be. pH Down is fast-acting, colorless, easy to use, and will not cloud aquarium water. Because it is phosphate-free and nitrate-free, pH Down pH will not encourage algae growth.