Want a quick and easy way to maintain calcium levels in your aquarium? Try adding a canister filled with aragonite-based media to your RO/DI filter. The water will pick up a little bit of calcium as it passes through the media, which will therefore also add calcium to the aquarium every time you top-off water or do a water change. The calcium-filled media chamber should be placed after all of the stages in the RO unit, as you don’t want to exhaust any of the filters prematurely. Also, you might get a little extra phosphate in your aquarium if you go this route, so you might have to incorporate a reactor with some type of phosphate removing media (e.g. GFO, biopellets, etc).