Magnesium plays a vital role in reef aquariums: it helps maintain calcium and carbonate (composing the majority of measurable alkalinity) concentrations by limiting unwanted inorganic precipitation with each other. In the absence of adequate magnesium ions, calcium and carbonate ions bond rapidly, depleting their concentrations in the system and creating the false impression that heavier supplementation is required to re-establish the desired concentrations. No matter how much calcium is added, its natural seawater concentration of 412 ppm will never be sustained until the magnesium concentration begins to approach 1, 290 ppm. Once the balance between calcium and magnesium has been established, subsequent calcium supplementation yields immediate positive results. Keeping magnesium within a range of 1, 290-1, 320 ppm will maintain the desired balance, and provides enough of the element to reef-building organisms for the formation of aragonite (~0. 10% Mg by weight).