Pick the product you actually own. Each page carries that product's verified potency, so the answer is an exact number of millilitres for your tank rather than a generic cap-per-gallon. 20 products across alkalinity, calcium and magnesium, every potency cited.
Open the full calculatorThis is the mistake that makes a correct calculation come out wrong. A tank labelled 75 gallons does not hold 75 gallons of water once rock, sand and displacement are accounted for, and the sump adds some back. Net volume is typically 10 to 20 percent below the number on the box. Dosing against the label figure overshoots every single time, quietly, in the same direction.
Every dose here is calculated from a current reading and a target. A dose calculated from last week's number is a guess. Titration kits carry real error, so treat a single reading that disagrees sharply with the trend as suspect and test again before making a large correction.
Alkalinity should not be raised more than 1.4 dKH in a day, calcium more than 50 ppm, magnesium more than 100 ppm. Larger corrections are split across days automatically. Corals tolerate a slightly wrong number held steady far better than a correct number arrived at abruptly.