pH is the measure of the concentration of free acid, hydrogen ions H⁺, in the water. Hydrogen ions is what an acid releases into the water. So the higher the concentration of hydrogen ions, the more acidic the water it.

pH is a a logarithmic scale, which means that changing the pH by one unit equates to a 10 fold change in hydrogen ion concentration. The actual relationship is pH = -log₁₀[H⁺] or [H⁺]=10^-pH. For example at pH 7 the concentration of hydrogn ions is 10^-7 M and at 8 it is 10^-8 M.

The scale is actually open ended, meaning that it can take on any value. However, the range found in the real world is from around 0 to 14. pH 7 is called neutral, <7 is acidic and >7 is basic or alkaline.

Note that it is lower case "p", followed by an upper case "H".

The pH of a system is influence by the generation or addition of acids and bases, alkalinity level, gas exchange, photosynthesis and metabolism.

The preferred range is 7.8 to 8.5 for reef aquariums, if alkalinity sufficient (8.0 to 8.3 is the typical value for natural salt water).

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