Hard corals have a calcium carbonate skeleton and will drain your dosing containers much faster than soft corals! In order to keep these, you will want to be well aware what your alkalinity, calcium, and possibly even magnesium levels are. Remember, stability is key, and having perfect levels is a lot less important than having stable levels. “Hard corals” are a group of corals broken down into two classifications, small polyp stony (SPS) and large polyp stony (LPS). The care required for these can vary considerably, with SPS corals needing more light and flow, and many LPS enjoying more moderate flow and lower to moderate light. All hard corals are within the order Scleractinia, and then further separated into family, genus, and finally species.