The concentration of magnesium in natural seawater reaches roughly 1200 mg/l and similar levels should be kept in marine aquariums with fish and invertebrates. Suitable magnesium content is essential for several biochemical processes occurring in the cells of marine organisms. Calcium needs to be in balance with magnesium, otherwise, invertebrates, especially corals, are not able to absorb calcium from the water to construct their skeletons. Decorative red calcareous algae are particularly sensitive to magnesium deficiency. The lack of this microelement contributes to the general deterioration of red algae, the loss of colour and growth inhibition. Depletion of magnesium by biological and chemical processes takes place continually in a fish tank, therefore, regular monitoring and supplementing of this element is necessary.