API pH Down is a liquid pH lowering product for freshwater aquariums. pH Down lowers pH by neutralizing carbonate hardness in the water. While many common aquarium fish will live in water with a pH range of 6.8 to 7.5, a pH of 7.0 is ideal for a community aquarium containing a variety of freshwater fish. Live bearers like mollies and sword tails prefer a higher pH of 7.2-7.6. Acid-loving fish such as angelfish and neon tetras prefer a lower pH of 6.5 - 6.8.