Mg Magnesium Test for aquarium. Magnesium is a major salt in natural seawater. Contrary to its significance this element was ignored for a long time. We know today that Magnesium creates a remarkable improvement in the growth of calcareous algae and invertebrates. In Marine Aquariums it is good practice to keep its concentration around 1.300/1.350 mg/lt. otherwise aqueous calcium ions (Ca2+) could bond with carbonates and not be available for corals to intake. ELOS AquaTest Mg permits the precise measurement of Mg values, thus permitting to perfectly dose our Magnesium additives. Grading: each drop equals 50 mg/Lt of magnesium. Good for 50-60 tests depending on magnesium levels.