Dosing calculator
How much to add to move alkalinity, calcium or magnesium from where your tank is to where you want it — worked from molar masses you can check, with the daily limits that matter more than the dose itself.
What you're correcting
The parameter corals notice fastest, and the one worth moving slowest. A tank sitting stable at 7.5 dKH is in better shape than one being walked up and down around 9.
Most reef tanks sit around 7.0–11.0 dKH. Stable matters more than any particular number in that range.
Use your actual water volume here, not the tank's advertised size — rock and sand displace a real system by roughly 10–20%, and dosing the advertised number overshoots by about that much.
What you're dosing with
Raises alkalinity with the smallest pH disturbance of the two, which is why it is the usual choice for a tank that is already low on pH.
That is the total for the whole correction — do not add it at once. Alkalinity is the parameter corals notice fastest, and the published ceiling is 1.4 dKH a day. Split this over 2 days — about 0.8 dKH, or 7.7 g, per day.
How this number is worked out (and the parts worth double-checking)
The dose comes out of molar masses, not a lookup table: Sodium bicarbonate (NaHCO₃) needs 0.03000 g per litre per dKH, from (1 ÷ 2.8 meq per dKH) × 84.007 g/mol ÷ 1 eq/mol ÷ 1000. Multiply by your correction and your water volume and you have the figure above. Every step is checkable against a periodic table, which is exactly why this tool does the chemistry rather than reprinting a brand's dosing chart.
The arithmetic is exact; the inputs are where the error lives. A hobby test kit is typically good to about ±0.5 dKH or ±20 ppm, which for a small correction can be bigger than the correction itself. Test before and after, and treat a single surprising reading as a reason to re-test rather than to dose.
Alkalinity's published daily ceiling is 1.4 dKH (Bulk Reef Supply's own calculator uses 1.5); many experienced reefers work to 0.5 dKH a day instead, and nobody has ever regretted going slower. Calcium and magnesium have no equally crisp published ceiling, so this tool does not invent one — it tells you to split a large correction rather than printing a number it cannot cite.
Two things this calculator cannot see. Calcium and alkalinity are consumed together as corals build skeleton, so a tank low in both wants a balanced two-part, not one element corrected in isolation — repeatedly fixing one alone drifts the tank's ionic balance, and a water change is the honest reset for that. And never add the two halves of a two-part at the same time or in the same spot: calcium and carbonate meeting at full strength precipitate straight back out as chalk, taking both out of the water instead of putting either in.