Alkalinity (also called carbonate hardness or dKH) measures the buffering capacity of your water — its ability to resist changes in pH. In a reef tank, alkalinity is primarily consumed by stony corals as they build their calcium carbonate skeletons.
Why Alkalinity Matters
Without adequate alkalinity, corals cannot grow and their existing skeletons can dissolve. Swings in alkalinity (even between two "acceptable" values) cause more stress than a stable slightly-off reading. Stability is paramount.
Target Range
Most reef keepers aim for 8–9.5 dKH. SPS-dominant tanks often run 7.5–8.5 to prevent potential issues at higher levels. LPS and soft coral tanks can tolerate 9–11 dKH.
How to Maintain Alkalinity
There are several methods to replenish alkalinity consumed by corals:
- Two-Part Dosing — separate calcium and alkalinity solutions dosed in equal amounts
- Kalkwasser — limewater that simultaneously raises both calcium and alkalinity
- Calcium Reactor — uses CO2 to dissolve aragonite media; ideal for large, heavily stocked systems
- Water Changes — fresh saltwater restores depleted elements naturally
Testing
Test alkalinity at least twice a week in an active reef. The Hanna checker (HI755) is a popular digital option. Salifert and Red Sea test kits are reliable manual options. Always test at the same time of day, as pH fluctuations affect readings.