Magnesium is present in natural seawater in a fairly high concentration (1300 - 1400 mg/L).
Magnesium is an essential part of chlorophyll, which is necessary for photosynthesis.
Without photosynthesis plants, including algae and the corals, would not be able to live.
Maintaining a correct magnesium concentration is very important, and is indirectly responsible for fast coral and calcareous algae growth by virtue of making the maintenance of correct calcium and alkalinity figures possible.
Magnesium is depleted by algae and is also depleted by the use of excessive Kalkwasser.
There are also certain brands of salt, which have very low magnesium content. Use of such a salt will result in permanent problems with calcium and carbonate hardness values.
The Salifert magnesium test is very straight forward, and does not suffer from calcium and strontium interferences when within certain bounds.
It measures in sufficient accurate steps of 30 mg/L with a sharp color change.
*Discounts exclude products from these manufacturers with MAP (minimum advertised price) rules: Ecotech Marine, Neptune Systems (Apex), Coral Vue (Reef Octopus, Maxspect), Aqua Illumination and Aqua UV.