Calcium Reactor Guide

How a calcium reactor uses injected CO2 to dissolve calcium-carbonate media and replenish calcium and alkalinity in reef aquariums.

What it is

A calcium reactor is a device that replenishes the calcium and alkalinity consumed by reef organisms. Stony corals build calcium-carbonate skeletons and draw calcium and carbonate from the water, so in a closed reef system these have to be supplied continuously.

How it works

The reactor canister is filled with calcium-carbonate media such as crushed coral skeletons. Carbon dioxide is injected into the canister, acidifying the water inside and dissolving the carbonate media. The resulting acidic, calcium-rich solution is then pumped into the sump, where it raises the calcium level and alkalinity of the system water.

Role in reef chemistry

Reef husbandry calls for stable and optimal calcium, carbonate and pH levels, with calcium commonly maintained around 400 to 450 ppm and alkalinity around 7 to 12 dKH. A calcium reactor is one of the established supplementation methods, alongside kalkwasser (a calcium-hydroxide solution in which the calcium hydroxide combines with dissolved CO2 to form calcium carbonate) and manual dosing of calcium and buffer additives, which are often tracked and adjusted with test kits and small dedicated controllers that monitor water quality.

Where it fits

Calcium reactors are associated with reef systems run on approaches such as the Berlin method, which relies on live rock for biological filtration together with protein skimming and calcium supplementation. They are most relevant where coral demand for calcium and alkalinity is high enough that water changes or hand dosing cannot keep up.

Choosing and sizing

The reactor is sized to the calcium and alkalinity demand of the livestock rather than tank volume alone, since a heavily stocked stony-coral system consumes far more than a lightly stocked one. CO2 injection rate and effluent flow are adjusted together so the dissolved output matches consumption, and parameters are tracked with test kits or automated monitoring.

Maintenance

The carbonate media is topped up as it dissolves over time, and the CO2 supply, effluent line and feed pump are checked so the dosing stays steady. Because the effluent is acidic, its rate and the resulting system pH and alkalinity are monitored to avoid overshooting or depressing pH.