What raises calcium fastest in a reef tank?

Calcium chloride solution. Per Randy Holmes-Farley recipe 1 (500 g dihydrate to one gallon), 1 mL per gallon of tank water raises calcium about 9.8 ppm.

Why will my calcium not stay up?

Usually low magnesium. Under about 1250 ppm magnesium, dosed calcium precipitates with carbonate instead of staying dissolved. Correct magnesium to 1250-1350 ppm first.

ReefMath

How to Raise Calcium in a Reef Tank

Calcium chloride is the fastest fix; two part systems maintain it. Never raise more than 50 ppm per day, and if calcium will not hold, test magnesium before dosing more.

Open the full calculator

Steps

Test magnesium first: below 1250 ppm, calcium will not stay in solution. Fix magnesium, then calcium.
Calculate the exact dose for your water volume with the calculator below.
Dose max 50 ppm per day into high flow, away from coral tissue.
Retest after 30 minutes of mixing before any second dose.