API Marine Magnesium Raises Magnesium Levels in Reef Aquariums raises and restores magnesium levels in your reef aquarium, and should be used whenever a magnesium deficiency is present in your water. The magnesium level of a reef aquarium should be between 1,250 to 1,450 ppm. Use API Marine Magnesium when water tests show a magnesium level below 1,250 ppm mg/L in your reef aquarium. Dose according to directions on the product label. Treats up to 948 US gallons of salt water.