Ocean water carries about 420 ppm calcium. Reef tanks do well anywhere from 380 to 450 ppm; corals and coralline algae consume it constantly alongside alkalinity in a roughly fixed ratio.
Open the full calculator| Tank type | Target |
|---|---|
| Natural seawater | 420 ppm |
| SPS dominant | 400-450 ppm |
| Mixed reef | 410-440 ppm |
| LPS / soft coral | 380-440 ppm |
Raise no more than 50 ppm per day. If calcium keeps falling while alkalinity holds, check magnesium first: below about 1250 ppm magnesium, calcium and alkalinity refuse to stay in solution together.