Alkalinity, also referred to as carbonate hardness or buffering capacity, is essentially a means of quantifying the stability of pH in a solution, such as aquarium water. In an aquarium, weak acids formed by oxygen respiration and the decomposition of organic material gradually deplete alkalinity.

If left unchecked, the alkalinity drops below a critical level and pH begins to fluctuate more than is normally suggested (a daily pH shift of +/-0. 2 is normal in marine aquaria). This would cause stress to livestock. Alkalinity is also depleted by reef-building organisms during the production of aragonite (~60% carbonates by weight), and is very important for their rapid growth.

Maintaining the alkalinity in marine aquaria between 7 - 12 dKH (2. 5 - 4. 3 meq/L) will generally maintain pH within the desired range, and Brightwell Alkalin8. 3P makes that easier.