Calcium Hydroxide Kalkwasser Powder 3.5 lb : 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.