The element™ Ca Reactor provides and maintains a virtually constant calcium, magnesium and alkalinity ratio, which is critical to the long term success of reef aquaria. The element™ Ca Reactor functions by using CO2 to react with a calcium reactor media such as Seachem’s Reef Reactor™. CO2 acidifies the water flowing through the reactor and thereby dissolves the media which releases calcium, magnesium, carbonates among other elements and components of the chosen reactor media. A CO2 filled tank along with a pH probe and preferably a CO2 controlling system are required and/or highly recommended for proper functionality of the element™ Ca Reactor.