Reef Dosing Calculator

Both raise alkalinity through the same basic chemistry, but soda ash (sodium carbonate) is generally preferred in reef tanks because it adds less sodium per unit of alkalinity boost than baking soda (sodium bicarbonate) does, which matters for long-term water chemistry balance. Baking soda remains a widely available, effective alternative many hobbyists use, especially in a pinch, which is why both dosing amounts are calculated here.

It's a rough proportional estimate tied to your calcium dose rather than a calculation based on an actual magnesium reading, so treat it as a rough starting reference rather than a precise dose. Magnesium plays an important role in keeping calcium and alkalinity from precipitating out of solution together, so if you're running a mature reef tank, testing magnesium directly and dosing to a specific target (typically 1250-1350 ppm) will give a much more reliable result.

Calcium and carbonate (the basis of alkalinity) are the two ingredients corals combine to build their calcium carbonate skeletons, so both get consumed together as coral growth happens, and dosing only one while ignoring the other risks pushing their ratio out of balance. Reef keepers typically track both calcium and alkalinity together, along with magnesium, since all three interact and an imbalance in one can affect how stable the others are in solution.

A dose that raises calcium by a certain amount in a small tank will raise it by far less in a large one, because parts-per-million is a concentration measure — the same mass of supplement is diluted across more water in a bigger system. This is why the calculator asks for total system volume including your sump, not just the display tank, since dosing based on display volume alone would systematically underdose a system with substantial sump water.