You need both. The calcium reactor holds araganite material. The CO2 feeds the calcium reactor chamber and creates calcium carbonate i think. However it works and someone can explain better than I, if it is set up properly it perfectly doses calcium,carbonates, and maintains a stable ph eliminating the need to dose said items. It just has an incredibly huge startup price and needs to be meticulously calibrated. But i personally think its WELL worth the money in larger systems.