the CO2 is what creates the calcium. CO2 tank is controlled by a solenoid and a regulator. When CO2 is combined with saltwater, it creates carbonic acid. The carbonic acid which is circulated through the reactor full of crushed coral, breaks down the crushed coral thence releasing calcium (what coral is made of). Its best to have a Kalk reactor also because the CO2 lowers the Ph. To counter this, kalk is used which has a Ph of around 12. My Kalk is implemented in with my ATO.
Casey