Corals build aragonite skeletons from calcium and carbonate. Natural seawater has about 420 ppm calcium, and most reefs run 400–450 ppm.
Calcium and alkalinity are linked
Calcification removes calcium and alkalinity in a fixed ratio: about 20 ppm calcium per 1 meq/L (2.8 dKH), or roughly 7.1 ppm Ca per 1 dKH (Randy Holmes-Farley, "A Simplified Guide to the Relationship Between Calcium, Alkalinity, Magnesium and pH," Reefkeeping Magazine). If your alk drops 1 dKH a day, your calcium should drop about 7 ppm a day. When the numbers don't fit, look for test error, an unbalanced additive, or precipitation.
Why won't my calcium rise?
- Low magnesium lets calcium carbonate precipitate (magnesium guide).
- Alkalinity too high: pushing both high causes "snowstorm" precipitation.
- Test error: calcium titrations are sensitive to endpoint reading; compare to fresh saltwater.
How to raise calcium
Reef-grade calcium chloride dihydrate: about 3.67 mg per liter per 1 ppm. Raise no more than ~50 ppm per day, dosed at a different time from alkalinity. Example: 75 gallons +50 ppm ≈ 52 g CaCl₂·2H₂O. Calculator: dosing calculator. For long-term maintenance use 2-part or kalkwasser (2-part guide).
Can calcium be too high?
Above about 500 ppm, there's no benefit and precipitation risk rises if alk is also high. Let it fall naturally by pausing the calcium part.
Sources
FAQ
Can I dose calcium carbonate? No. It barely dissolves in tank water; it's media for calcium reactors.
Should I dose calcium and alk together? Not at the same time and place; separate by hours and into high flow.