Magnesium is an important element that is often overlooked by the beginner saltwater reef aquarium hobbyist. It is the third most abundance element in natural seawater after sodium and chloride and is found in concentration of around 1,280 ppm in natural sea water. Magnesium is absorbed in very small amount by corals and other invertebrates such as coralline algae, snails and clams along with calcium carbonate for the healthy building of the structures of the corals and invertebrates. As such, maintaining an optimum level of magnesium in your saltwater reef aquarium is important especially if you are keeping an aquarium with lots of fast growing stony corals or SPS corals. Even if you are not keeping lots of corals, it is desirable to maintain an optimum level of magnesium in your tank as it helps to slow down the calcium precipitation process and making the job of maintaining a more stable pH, alkalinity and calcium levels easier. Visit this page for more information about magnesium and how to control it in the saltwater reef aquarium.