The Calcium, Magnesium, and Alkalinity Triangle Explained

Why you can't manage these in isolation

Calcium and alkalinity are consumed together by calcifying corals and coralline algae — but they don't deplete at a fixed ratio, and magnesium acts as a buffer that keeps calcium and alkalinity from precipitating out of solution when either runs high.

The target ranges that matter

• Alkalinity: 7–10 dKH (most SPS-dominant tanks run 7.5–8.5 for stability)
• Calcium: 380–450 ppm
• Magnesium: 1250–1350 ppm

The trap most reefers fall into

Dosing calcium and alkalinity aggressively while ignoring magnesium eventually causes both to become harder to maintain — low magnesium reduces the water's capacity to hold calcium and carbonate in solution simultaneously, so you end up chasing numbers that won't stabilize no matter how much you dose.

If you're dosing calcium and alkalinity consistently but both keep reading low despite correct dosing math, check magnesium first. Raising magnesium back into range often resolves an alkalinity/calcium problem that looked like a dosing problem.

The practical takeaway

Test all three together, not just the one you're actively dosing. A monthly magnesium check alongside your weekly alk/calcium routine catches this exact failure mode before it becomes a multi-week troubleshooting rabbit hole.