The pH value or “degree of acidity” gives indications on how acidic or basic (alkaline) a water can be. It is a measuring instrument for the concentration of the acidic or basic components of an aqueous solution. Pure water has a neutral reaction and has a pH value of 7. If the acid components are present in higher quantities, the pH value is lowered to levels below 7. If the basic components exceed, the pH value is rises and goes to values above 7.
In freshwater aquariums, the optimum range for the pH value is between 6.2 and 7.2 (excluding cichlids) for most aquarium fish. Many organisms react extremely sensitively to large fluctuations in the pH value. Too abrupt lowering or raising should definitely be avoided.