pH is the measure of acidic or alkaline conditions of water. 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. Check the pH level using apH 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 lowers pH and makes aquarium water more acidic. pH DOWN is fast-acting, colourless, easy to use and will not cloud or colour the aquarium water. pH DOWN is phosphate-free and nitrate-free therefore will not encourage algae growth.