I use reef crystals. It's the same as IO but formulated more for reef tanks with higher Ca, Mg, and alk. I've heard oceanic often has bad batches with extremelly high Ca, >600 ppm. I wouldn't trust oceanic with my tank. IO is ok, but if you have a reef and want higher Ca, Mg, and alk you should switch to reef crystals. Or if you have money to spare tropic marin pro reef would be a better choice. The levels are similar to --, but -- uses chelated Ca, which is harder for corals to use. Tropic marin uses non-chelated Ca.