Calcium, Alkalinity, and Magnesium: Reef Tank Dosing Explained

Stony corals build their skeletons from calcium carbonate, which means three interrelated parameters — calcium, alkalinity, and magnesium — need consistent attention in any reef tank with LPS or SPS coral.

Calcium

Target roughly 380-450 ppm. Calcium is consumed continuously by growing coral, so tanks with heavy coral stocking need regular dosing or a calcium reactor to keep pace.

Alkalinity

Measured in dKH, alkalinity represents the water’s buffering capacity and is arguably the most important parameter to keep stable, since swings here stress coral more than most other factors.

Magnesium

Magnesium (target 1250-1350 ppm) helps keep calcium and alkalinity in balance — low magnesium can make it difficult to maintain proper calcium and alkalinity levels no matter how much you dose.

Dosing Methods

Options range from manual two-part dosing to automated dosing pumps and calcium reactors. Whichever method you choose, test regularly and adjust gradually — consistency matters more than hitting an exact target number. See our Complete Guide to Coral Reef Tanks for the bigger picture.