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A poly reactor can be used for many different medias such as carbon, GFO, bio-pellets, etc, hence the word "poly". A carbon reactor is the same but just advertised to run carbon. A calcium reactor is an entirely different thing. It's a setup basically made to melt calcium carbonate by lowering the pH through the injection of co2 into it. It's used to maintain calcium and alk levels in a reef tank system.