Calcium reactors or calcium carbonate reactors have been in the hobby a long time, our German hobbiest counterparts have been using them since at least 1994 to support their SPS tanks.
Many people including myself try to maintain calcium and alkalinity levels in their tanks with kalkwasser and/or two part additives. And for many people this works very well. Eventually each of us progress into the world of Tridacnid clams and SPS corals and start having trouble keeping their calcium and alkalinity levels elevated. And as someone who has made 3 to 5 gallons of kalkwasser every single night its a relief to get away from it. The most basic way to describe how a calcium reactor works is this: It is the reverse process of calcification. Carbon Dioxide gas is used to create carbonic acid which dissolves a calcium carbonate media. This in turn creates calcium ions (Ca++) and bicarbonate ions (HCO3-). Best of all its added in a balanced proportion that is used in calcification.
The formula commonly used to describe this process is:
CaCO3 + H2O + CO2 <-> Ca2+ + 2HCO3-