API pH Down, 16 OZAPI pH DOWN pH adjuster lowers pH to make freshwater aquariums more acidic for fish and plants when levels rise too high. pH is the measure of acidic or alkaline conditions in water, where a value of 7. 0 corresponds to a neutral pH, pH values about 7. 0 are considered alkaline, and pH values below 7. 0 are considered acidic. Different fish need different pH levels to be healthy, so it is important to test your water regularly to ensure pH levels fall