You dont have to add calcium to your tank with a fowlr. Water changes will replemish enough calcium and trace elements to sustain your coraline on your rocks. If you are looking for more coraline growth you can add calcium to your tank. Right now I add nothing to my tank. I do weekly waterchanges of 10% and Everthing seems to do great. Iam not having a ton of coraline growth but it maintains what I have. I only keep soft corals like mushrooms and zoos the easy stuff for right now. In the future when I start to get into the hard corals then is when you have to start adding additives.HTH