Alkalinity is the parameter corals notice fastest and the one that punishes speed hardest. The arithmetic below is the easy half. The number that matters is the second one: how far you can move it in a day without the correction costing you tissue.
Worked from your tank's real water volume rather than the number on the box. The calculator itself needs JavaScript; everything it assumes is written out below.
A swing is worse than a wrong number held steady. Corals adapt to an alkalinity they do not like far better than they adapt to one that keeps moving, which is why this calculator spreads a correction across days rather than handing you one dose.
Soda ash raises pH along with alkalinity and sodium bicarbonate barely moves it. In a tank already sitting high on pH in the afternoon, that difference decides which of the two you should be reaching for.
If alkalinity will not hold no matter what you dose, the answer is often not more alkalinity. Low magnesium lets calcium and carbonate precipitate about as fast as you add them, so it is worth reading before you chase this one for weeks.
Between 8 and 9 dKH for most reef tanks, and the calculator's targets default to that. Natural seawater sits near 7 dKH, but a closed tank runs it a little higher for headroom, because alkalinity is consumed every hour corals calcify and nothing tops it up between doses. Anything from 7 to 12 dKH can be kept stable; what the calculator refuses is a target outside that, because no additive schedule makes it safe.
No more than 1.0 dKH a day. The calculator spreads a larger correction over as many days as it needs rather than handing you one dose, because moving alkalinity quickly burns coral tissue even when the final number is right. A tank at 6.5 heading for 8.5 is a two-day job at the very least, and a slower one is fine.
One degree of carbonate hardness is 17.85 ppm (mg/L) of calcium carbonate equivalent, or 0.35666 meq/L. So a reading of 8 dKH is about 143 ppm, and a test kit that reports in meq/L is read by dividing by 0.35666 to get dKH. The calculator works in dKH because that is what most reef kits print.
Per dKH of rise, soda ash (sodium carbonate) takes 1.89 g per 100 litres and baking soda (sodium bicarbonate) takes 3.00 g, because carbonate supplies two equivalents of alkalinity per molecule and bicarbonate one. Soda ash also pushes pH up and bicarbonate barely moves it, so a tank that already runs high on pH in the afternoon should be on bicarbonate, and one that runs low gets both jobs done by soda ash.
The rules that apply to every dose rather than to this one, including never dosing calcium and alkalinity together, are on the <a href="/tools">calculators hub</a>.