Magnesium is the 3rd most abundant element in saltwater. We will concentrate on the interaction between magnesium with calcium and alkalinity balance in reef aquarium. Calcium and magnesium are structurally much alike. Corals use Mg2+ by incorporating it into their skeletons. We know that Ca2+ and alkalinity have an inverse relationship. Mg2+ provides buffering capacity by bonding with carbonate ions. Mg2+ binds with Ca2+, it allows for more hardness in the water than if Mg2+ were not there. High Mg2+ levels allow for the addition of Ca2+ without the drop in alkalinity.The natural concentration of Mg2+ in seawater is approximately 1285 ppm. Most salt mixes have a good amount of Mg2+ already. Consistent water changes with reed salt will raise Mg2+ levels. Alternatively, you can dose a magnesium supplement. Talk with your Pisces Reef fish specialists to see what you are needing.