Alkalinity is a measure of the pH buffering capacity of water. It indicates the concentrations of carbonate (CO3)2-, hydrogen bicarbonate ( HCO3 )-, borate ( BO3 )3-, sulphate ( SO4 )2-, and hydroxide ( OH )- anions. The actual value is determined by the amount of free acid, hydrogen ions ( H+ ), required to neutralize all of the above anions.

This is what provides the correct and stable pH for a reef aquarium, if maintained at sufficient levels. A correct and stable pH. non fluctuating one is important for the health of the aquariums inhabitants. It is true that alkalinity is important as it is a measure of the ability to resist a drop in pH but the water works in both directions. Some components of the alkalinity buffer system are also utilized by organisms, such as hard coral, fish, algae growth and etc in order to maintain a sufficient amounts for good health and growth. Additionally, the higher the alkalinity, the greater the ability of the system to absorb the addition of an acid or a base with only small change in the actual pH. Alkalinity levels of 8.2~8.4 are recommended to keep a stable pH in a reef aquarium.

A buffer is a series of chemical species in a solution that resists a change in pH when either a base eg. hydroxide ions ( OH- ), or an acid, eg. hydrogen ion ( H+ ), are added to a solution. It does this by acting as a reservoir for H+, donating them to solution when the concentration fail and taking them from the solution when the concentration rises.

The buffering system involves a base and an acid, in relatively high concentrations, inequilibrium with each other. The base acts as a hydrogen ion absorber and the acid as a hydrogen donator. When this equilibrium is upset by the addition of H+ or OH+, which in effect removes H+, then the acid and base alter their concentrations until equilibrium is again achieved. When this equilibrium is retained the pH is close to the original pH.