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.