Stony corals and coralline algae pull calcium and carbonate from the water to build skeletons. Alkalinity measures carbonate availability; calcium and magnesium are the other legs of the stool. If one is low, dosing another alone often fails.

Typical targets many keepers use: alkalinity 8 to 9 dKH, calcium 400 to 450 ppm, magnesium roughly 1250 to 1350 ppm. Your corals and salt mix may favor slightly different numbers — stability beats chasing perfect lab values.

Test alkalinity at least weekly on stocked reef tanks; calcium and magnesium weekly or biweekly. Log all three on the same day in ReefDiary so you see when alkalinity drops while calcium looks fine — a common clue that dosing balance is off.

Alkalinity, Calcium, and Magnesium in Reef Tanks