Measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH around 7.0, solutions with pH of less than 7.0 acidic and greater than 7.0 basic or alkaline. Measured on a logarithmic scale of 0.0-14.0 whereby a change of a single unit represents a ten-fold increase in acidity or basicity/alkalinity, i.e., a solution with a pH of 4.0 is ten times more acidic than one of 5.0., and 100 times more acidic than one of 6.0. Some fishes require a relatively specific pH in aquaria, while others are more generalised, with the former requirement often-reduced in captive-bred examples.