pH is the measure of acidity of water, on a scale ranging from 0 to 14. A pH of 0 is the most acidic, while a pH of 14 is the most alkaline. Different geographies have different tap water conditions with different pH levels, while fish require a specific pH level to thrive. A pH of 7.0 is ideal when keeping a community aquarium containing a variety of tropical fish. Goldfish and livebearers prefer a pH of 7.5. Many Amazonian fish, like angelfish and neon tetras, prefer a pH of 6.5 to 6.8. Mollies and swordtails thrive at pH 7.2 to 7.5. API pH DOWN Freshwater Aquarium Water pH Reducing Solution lowers aquarium pH in freshwater aquariums to desired level. Safe for fish and plants. Use when setting up a new aquarium, adjusting pH, or changing water. pH in an aquarium can fluctuate due to fish waste, uneaten food and addition of tap water. It is recommended to test water for pH weekly. With API Aquarium products, its easy to keep a beautiful saltwater, freshwater or reef aquarium. For over 50 years, API has been creating innovative, research-driven solutions that make it easier to care for your fish and aquarium. API Aquarium Treatment Supplies are designed to work in conjunction with each other to provide best results to control algae, promote healthy bacterial growth and help control and cure fish diseases and conditions such as ich and fin rot. They work to provide a safe, hospitable environment for fish such as tropical community fish, cichlids, goldfish and more. With API, you can spend more time admiring your fish, and less time scrubbing your tank. Make sure to use all products as directed in order to ensure the best results. Ingredients Suitable For Both Freshwater And Saltwater Aquariums. Some Water Is High In Mineral Ions Such As Carbonate And Bicarbonate, Which Strongly Buffers Water And Can Resist Ph Adjustment. Repeated Dosing May Be Needed To Lower Ph. Contains No Phosphates. Directions Determine the pH of aquarium water with an API pH TEST KIT. To lower pH, add 2 drops per U.S. gallon (3.8 L) of aquarium water. Take another pH reading before adding another dose. Note that some fish may be sensitive to pH adjustments greater than .2 in a 24 hour period, so changes of this magnitude she be executed with extreme care. Use the API pH TEST KIT weekly to accurately monitor pH. Then, when necessary, use pH DOWN to reduce pH levels in your aquarium. Tip: Some water is high in mineral ions, such as carbonate and bicarbonate, which strongly buffer aquarium water and can resist a pH change or cause pH rebound. In these situations, repeated dosing may be necessary to lower pH.