API pH Down 118mlpH is the measure of acidic or alkaline conditions of water. A value of 7.0 corresponds to a neutral pH. pH values above 7.0 are considered alkaline, and pH values below 7.0 are considered acidic. Different fish need different pH levels to be healthy. Check the pH level using a pH Test Kit. pH Down is designed to lower the pH of aquarium water where testing has shown pH to be at an undesirable level. pH Down is fast-acting, colorless, easy to use and will not cloud or colour the aquarium water. it is also phosphate-free and nitrate-free therefore will not encourage algae growth.