pH is the measure of the acidity or basicity of a solution.
The pH scale runs from 0 (acidic) to 14 (alkaline), with neutral pH 7 in the middle. The pH scale is loarithmic, which means, for example, that pH 5 is ten times more acidic than pH 6. The pH range of interest to the tropical fishkeeper is between pH 5 and pH 9, with the vast majority of fish requiring a pH between pH 6 and 8. The pH value of the water has an important influence on the way a fishes body functions.