Magnesium Power Dupla Marin is the ideal magnesium supplement for the judicious care of sea water aquaria. Magnesium, one of the most important elements in sea water aquaria is required for healthy skeleton building of corals and calcareous algae. A sufficient magnesium level also prevents calcium precipitations. The magnesium content of natural seawater is approximately 1,350 mg/l. Invertebrates in aquariums may react negatively to significantly elevated magnesium levels. Magnesium and calcium must be balanced in a given proportion to each other. It is therefore necessary to regularly monitor both values. The optimum calcium level is 380 to 400 mg/l.