What Are Calcium and Magnesium?
Calcium and magnesium are the other two legs of reef chemistry alongside KH — together they’re often called “the big three.” Calcium is the main building block corals use for their skeletons; magnesium doesn’t get consumed nearly as fast, but it keeps calcium and carbonate (KH) from precipitating out of solution together, which is why letting it drop causes the other two to become unstable and hard to dose.
Why They’re Tested Together
Reef keepers typically target roughly 400–450 ppm calcium and 1250–1350 ppm magnesium, alongside 8–12 dKH — and the three are chemically linked, so a change in one affects how stable the others are. A common troubleshooting mistake is chasing a low calcium or KH reading with heavier dosing while magnesium is quietly low in the background, which limits how much calcium and carbonate the water can actually hold in solution.
Shop Calcium & Magnesium Test Kits
The Salifert Calcium and Magnesium test kits cover both, and the wider Marine Aquarium Water Conditioner range has dosing supplements for both parameters.
