Calcium carbonate reactors have become a popular way of replacing the calcium and carbonate taken up by corals in the process of calcification. In its most basic form, a calcium reactor is simply a container filled with calcium carbonate (CaCO3) media over which aquarium water is passed with the addition of carbon dioxide. Adding carbon dioxide lowers the pH of the water, making it acidic, and dissolving the calcium carbonate to provide the aquarium with calcium and alkalinity.
Why would I need a calcium reactor? Most people that grow stoney corals sooner or later discover that it's just too much work adding a calcium supplement 3 or 4 times a week. A calcium reactor adds in calcium and trace elements for your corals, so you don't have to! Let's face it, anything that makes reef tank maintenance less time consuming leaves more time for actual enjoyment, and if you value your time as much as I do, a calcium reactor will pay for itself in no time at all!