Calcium Hydroxide - is the proven standard for maintaining calcium concentration and alkalinity in closed marine aquaria. The calcium provided is directly used by calcifying organisms, and the hydroxide ions provided readily convert dissolved CO2 into bicarbonate and carbonate ions, which are also directly used by calcifying organisms. The net result is no ionic imbalances in the aquarium water, which can occur through the prolonged use of other calcium supplements, such as calcium chloride. This product is extrmely pure and meets American Water Works and Water Chemicals Codex Specifications. This means it's pure enough to be used in low-levels as potable water additive by water municipalities. Due to the high pH of Calcium Hydroxide solutions, it's important to add slowly ("dripping" is best) to your system so as to prevent localized precipitation and the resultant loss of alkalinity.