Salifert Alkalinity Calcium Magnesium Combo Test Kit Alkalinity and calcium are needed to form the skeletal material of corals. Alkalinity and calcium are not the only substance needed to form the skeletal material of corals. Maintaining a correct magnesium concentration is very important and is indirectly responsible for coral growth. Magnesium is depleted by algae growth and also by the use of excessive kalkwasser. Magnesium is present in natural sea water in a fairly high concentration (1350 - 1500 mg/L) and is a Magnesium also helps to maintain the correct combination of calcium concentration and alkalinity. The Salifert Combo Test Kit is sensitive enough to detect small changes in levels of alkalinity, calcium and magnesium. The test kit is easy to use and accurate.