Ocean water carries about 1280 ppm magnesium. Keep reef tanks between 1250 and 1350 ppm. Magnesium is the stabilizer: when it is low, calcium and alkalinity precipitate out and refuse to hold their targets.
Open the full calculator| Tank type | Target |
|---|---|
| Natural seawater | 1280 ppm |
| All reef types | 1250-1350 ppm |
Raise no more than 100 ppm per day. Large corrections take days; that is normal. Magnesium is consumed slowly, so once corrected it usually holds with water changes alone.