CO2 systems in the marine aquarium are used to operate a chalk reactor. Unlike in fresh water where one talks about a CO2-fertilization; a CO2 system is required in a marine tank only to operate a chalk reactor. So CO2 is added to the reactor which is mostly filled with coral breakage to reduce the internal pH value. The functionality of the reactor is that in case of an acidic pH value under pH7, the calcium carbonate skeletons are dissolved and the individual elements are again dissolved in water which have stored the dead hard corals. This solution is then added to the aquarium. On our online shop you will find cheap CO2 systems, CO2 bottles, pressure regulator and complete systems.