Net water, not tank rating. Use the Tank Volume calculator if you have not measured it.
How much carbonate to add to move dKH from where it is to where you want it.
Raises 284 L by 0.70 dKH using Sodium bicarbonate, NaHCO₃. Add it to high flow away from corals, ideally to the sump return chamber. Retest after eight hours, not immediately.
dose = (8.50 − 7.80 dKH) × 283.9 L ÷ 33.33 = 5.96 g
Alkalinity is a capacity, not a concentration, so the arithmetic is linear. Adding n milliequivalents to V litres raises alkalinity by n/V meq/L regardless of where it started.
The unit chain: 1 meq/L = 50.043 mg/L CaCO₃ = 2.804 dKH. The hobby rounds that to 2.8, which is a 0.14 percent error, far below what any test kit resolves. This site uses 2.8.
Sodium bicarbonate at 33.33 dKH per gram per litre is pure stoichiometry, not a product claim. NaHCO₃ has a molar mass of 84.006 g/mol and supplies one equivalent per mole, so one gram is 11.904 meq. Multiply by 2.8 and you get 33.33 dKH in one litre.
Soda ash (Na₂CO₃, molar mass 105.988) supplies two equivalents per mole, so one gram is 18.870 meq, or 52.84 dKH per litre. That is why soda ash goes further per gram, and also why it pushes pH harder.
Baking sodium bicarbonate at 300 °F for an hour drives off water and CO₂ and converts it to soda ash, losing 37 percent of its mass. Weigh it after baking, then use the soda ash figure. Bake it if your pH runs low, use plain bicarbonate if your pH runs high.
The calcium figure comes from the mineral itself. Coral skeleton is CaCO₃, so one millimole of calcium pairs with one millimole of carbonate, which is two milliequivalents of alkalinity. That works out to 20.04 ppm calcium per meq/L, or 7.16 ppm per dKH.
The 1.4 dKH per day ceiling is a safety convention rather than a hard chemical limit. Large fast swings stress corals and can trigger precipitation that strips calcium and magnesium at the same time.
Sources: Seachem Reef Carbonate · Seachem Reef Builder · Brightwell Alkalin8.3 · Red Sea Foundation manual (PDF) · Dieck, Reef Chemistry Calculator