Magnesium plays a vital role in reef aquaria: 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). The rate at which magnesium is extracted from the water is determined by the stocking density of reef-building livestock, type of lighting, and other conditions; therefore, each aquarium will have different requirements for the rate of magnesium supplementation. Once the rate of magnesium uptake in the aquarium has been determined, the proper dosing rate of this product can be easily calculated. It is recommended that a quality salt mix with the proper (not augmented) alkalinity and concentrations of major, minor, and trace elements be used to establish natural seawater parameters in the reef aquarium, providing a stable ionic foundation on which to build. All Brightwell Aquatics liquid water care supplements are made with purified water and high purity materials. We develop formulae for aquaria based on empirical data, using observation and measurement rather than theory.