Dosing Calcium and Alkalinity in Reef Tanks
How reefkeepers replenish the calcium and alkalinity corals consume: two-part dosing, kalkwasser, calcium reactors, the Balling method, matching dose to uptake, automation and risks.
Overview
As stony corals and coralline algae grow, they remove calcium and carbonate (alkalinity) from the water, and in a stocked reef tank this happens faster than water changes alone can replace. Dosing is the routine replenishment of calcium, alkalinity and, as needed, magnesium so these parameters stay stable. Several methods exist; all aim to return calcium and alkalinity in the balanced proportion the corals use.
Two-part dosing
The two-part method uses two separate solutions, one supplying calcium and the other alkalinity, added separately so they do not react in the bottle or in a concentrated stream. Because the parts are dosed independently, the calcium and alkalinity can be tuned to the tank's needs. Two-part is simple and precise and is widely used on small to medium reefs.
Kalkwasser (limewater)
Kalkwasser is calcium hydroxide dissolved in purified water (limewater). It is a balanced supplement that raises calcium and alkalinity together, and it also raises pH, which can help tanks with low pH. It is usually delivered slowly as part of the evaporation top-off water rather than in concentrated doses, because adding it too fast causes precipitation. Its capacity is limited by how much water evaporates, so heavily consuming tanks may outgrow it.
Calcium reactor
A calcium reactor slowly dissolves calcium carbonate media inside a chamber by injecting carbon dioxide, which lowers the pH in the chamber and releases calcium and carbonate into a stream returned to the tank. It supplies calcium and alkalinity together in balance and runs continuously, which suits larger or fast-growing reefs, but it requires CO2 and careful tuning.
Other methods
- Balling method: a multi-part dosing scheme using calcium chloride, a carbonate solution and magnesium plus trace elements, designed to keep the overall ionic balance correct.
- All-in-one supplements: single products that combine calcium, alkalinity and magnesium, convenient for lightly stocked tanks.
- Water changes: a salt mix supplies calcium and alkalinity and is enough for low-demand tanks but not for a coral-dense reef.
Matching dose to consumption
Whatever method is used, the dose should match what the corals actually consume. Reefkeepers measure how far calcium and alkalinity fall over a day, then dose enough to return them to target and hold them steady. As the corals grow, consumption rises, so the dose is increased over time. Dosing pumps allow the daily amount to be split into many small additions for stability and automation.