In marine and reef aquariums, a calcium reactor is a device used to create a balance of alkalinity in the system. An acidic solution is produced by injecting carbon dioxide into a chamber with salt water and calcium rich media. The carbon dioxide lowers the pH by producing a solution high in carbonic acid, and dissolving the calcium media. The effluent is then returned to the reef aquarium where the calcium is consumed by organisms, primarily corals when building skeletons. A calcium reactor is one of the most efficient methods of supplying calcium to a reef aquarium. Although more commonly used with marine reef aquariums, calcium reactors are beginning to find use in elaborate freshwater and brackish aquariums where some species of freshwater clams, corals, and other invertebrates may need a constant supply of calcium.

The objective is to dissolve the calcium laden media to provide both bicarbonates HCO₃^- (alkalinity) and calcium (Ca⁺⁺) ions at the sames rate as consumed during calcification. Effectively dissolving the media requires an acidic pH. Saltwater typically has a pH of at least 7.8, so to reduce the pH carbon dioxide (CO₂) is used. The general reaction formula is: