Magnesium is present in natural sea water in a fairly high concentration (1350 - 1500 mg/L) and is an essential element of chlorophyll which is necessary for photosynthesis. Without photosynthesis plants, algae and coral would not be able to survive. Magnesium also helps to maintain the correct combination of calcium concentration and alkalinity as it slows down the formation of calcium carbonate which can absorb many important trace elements within the aquarium. Maintaining a correct magnesium concentration is therefore very important and is indirectly responsible for fast coral and calcareous algae growth by making it possible to maintain correct calcium and alkalinity figures. Magnesium is depleted by algae growth and also by the use of excessive kalkwasser or by going far beyond natural calcium, alkalinity and pH values. The Salifert Magnesium Profi-Test is easy to use and accurate with measurements in steps of 30 mg/L. Results are not affected by strontium and calcium interference. Can be used for marine water only.Calcium and strontium will not interfere when their total concentration is between 200 and 650 ppm. This is mostly the case.Up to 50 tests.