Calcium is one of the three major elements (alongside alkalinity and magnesium) that reef keepers must actively manage. Stony corals — both LPS and SPS — extract calcium from the water to build their calcium carbonate skeletons.
Target Range
The natural seawater concentration of calcium is approximately 420 ppm. For a reef tank, aim for 400–450 ppm. Levels below 380 ppm can slow coral growth; levels above 500 ppm risk precipitation with carbonate and can crash your alkalinity.
The Calcium-Alkalinity Relationship
Calcium and alkalinity are inextricably linked. They must be dosed in balance — raising one without the other leads to precipitation ("snowstorm") and depletes both. This is why two-part dosing and calcium reactors are designed to raise both simultaneously in the correct ratio.
How to Dose Calcium
- Two-part: Dose equal parts of a calcium and alkalinity solution; auto-dosing pumps make this seamless
- Kalkwasser: Limewater (calcium hydroxide) raises calcium, alkalinity, and pH while precipitating phosphate
- Calcium chloride: A quick fix for low calcium, but raises chloride over time — not suitable for regular use
- Calcium reactor: Best for heavy coral loads; dissolves aragonite media with CO2
Testing
Test calcium at least weekly with a reliable test kit (Red Sea, Salifert, or the Hanna checker HI758). Log your results and calculate daily consumption to determine the correct dosing amount for your system.