Magnesium is a chemical element with the element symbol Mg. Magnesium is extremely important for most reef-inhabiting species, foremost however for corals. Magnesium in conjunction with calcium is responsible for the alkalinity (KH), i.e. the hardness of water. In a salt water aquarium, magnesium is consumed in significantly lower amounts than for example calcium. Nevertheless it should be monitored regularly.
The magnesium content in the water is dependent on the salt concentration. The natural concentration of magnesium in salt water is 1,310 mg/l (which equals a salt content of 3.5%). The lower the salt concentration, the lower the magnesium content. In practice, a reef aquarium should have a 1:3 ratio of calcium to magnesium. In other words, a calcium value of 400 mg/l should equal a magnesium concentration of approx. 1200 mg/l.
The Giesemann magnesium test offers an uncomplicated, fast and precise measurement of the magnesium concentration in salt water aquariums.
By the way: many artificial reef ceramics also release magnesium into the water. Therefore, especially for newly equipped aquariums, it is essential to monitor the magnesium concentrations closely.