pH – The degree of acid in the water
The pH-value indicates the degree of acid in the water. A pH of 7 is neutral, below 7 is acidic and if over 7 the water is alkali. As the pH is to the power of ten (i. e. one-unit change means ten and two units mean 100) changing the pH must be carried out carefully to avoid any erratic changes.
There is no general advice for an optimum pH. Depending on their origin, ornamental fish have very different needs. For example fish from South America live in a very acidic environment (pH less than 7), however chichlids from East African inland waters live in alkaline water (pH more than 7).