Calcium Hydroxide (Kalkwasser Powder) Calcium Hydroxide is a 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. ESV's product is extremely pure and meets American Water Works Association and Water Chemical Codex requirements.