API pH Down Freshwater Aquarium Water Treatment, 4-oz bottle; Safely and quickly lower the pH in your freshwater aquarium for more acidic water conditions with API's pH Down Aquarium Water Treatment. Different fish need different pH levels to stay healthy. For example, most freshwater fish thrive in levels between 6.5 and 7.5, but Cichlids and crustaceans require higher pH levels between 7.8 and 8.8. Levels that are too high, or too low, can cause stress, illness and even lead to loss, so testing is crucial to the health of your fish. API pH Down is designed to lower pH in water that tests at an undesirable level. It is fast-acting, colorless and easy to use. It should not cloud or color aquarium water, and because it's phosphate- and nitrate-free, it should not encourage algae growth.