Magnesium is crucial to any saltwater tank that contains hard or soft corals. It plays a vital role in maintaining calcium concentrations. If the magnesium levels drop too low, it will be almost impossible to raise either the calcium levels or the alkalinity levels. Magnesium should be added in small amounts on a daily basis until the levels reach between 1200 and 1300 ppm to match the concentration found in seawater. Some magnesium supplements also contain iron and other elements.