Calcium basics
Reef tank calcium levels: a practical guide
Calcium supports coral skeleton growth, but a stable, tested system matters more than chasing one perfect reading.
What calcium does
Stony corals, clams and coralline algae use calcium as they build calcium-carbonate structures. Consumption changes as livestock grows, so yesterday’s dosing routine may not match today’s demand.
A useful target
Many reef keepers maintain calcium around natural seawater levels, commonly within roughly 400–450 mg/L. Choose a sensible target within the guidance for your salt mix and test kit, then favour consistency over frequent large corrections.
Testing frequency
Test more often while establishing consumption or after changing livestock, lighting, feeding or water-change routine. Once the system is stable, regular testing confirms that dosing still matches demand.
Read calcium with alkalinity and magnesium
These measurements interact. If calcium behaves unexpectedly, test alkalinity and magnesium before simply adding more calcium.