This is a photo of a Tridacna derasa clam in a 500-gallon reef tank owned by Greg Schiemer. Changing water efficiently in this tank is a serious concern. There are a couple of commercial magnesium supplements marketed for aquarium use that claim to provide magnesium along with anions in proportions resembling the anionic composition of natural seawater. Many people, deterred by the price of these supplements, have endeavored to make their own magnesium supplements. Typically, they use either pure magnesium chloride or magnesium sulfate.
Magnesium chloride is available in several grades, with varying amounts of water of hydration.
The most commonly encountered forms are magnesium chloride hexahydrate and anhydrous magnesium chloride. The physical and handling properties of dry magnesium chloride are superior to highly hydrated forms of this, to the joy of people putting it into spreading equipment. Anhydrous magnesium chloride is 25.53 percent magnesium by mass. The balance is made of chloride ions. Magnesium chloride hexahydrate has so much water that the magnesium is very nearly in solution already.