A calcium reactor consists of three main items, a chamber to hold the media, a circulation pump, and a carbon dioxide (CO2) kit (regulator & tank). The reactor is constantly fed CO2 to keep the water in the chamber at a lower pH. At low pH, the media (usually aragonite) dissolves releasing calcium, bicarbonate, and trace elements. The effluent from the reaction chamber is dripped slowly back into the aquarium where it acts to slowly boost the calcium level in the tank and also stabilize the pH and alkalinity. These reactors are not particularly good at quickly raising levels of calcium or alkalinity, but rather maintaining them once the desired level is achieved. Before running the reactor, it is a good idea to adjust the levels of calcium and alkalinity until you are satisfied.

A Nilsen reactor is a device named after Alf Jacob Nilsen, a decorated aquarist from Norway. It is an airtight a chamber partially filled with calcium hydroxide powder that is constantly fed water from a reservoir. It is important that this chamber is air tight since calcium hydroxide comes out of solution more easily if in contact with the atmosphere. A magnetic stirrer keeps the water in the reactor saturated by periodically stirring the powder. The kalkwasser (lime water) requires this occasional stir because it is not particularly water-soluble and precipitates if not regularly agitated. Unlike the calcium reactor, a Nilsen reactor does not use the aquarium's water for the reaction. It is fed water via a dosing pump from a container of fresh water, and saturates it with the dissolved calcium hydroxide. That saturated effluent is then administered into the tank as top-off water. The drawback to this method of calcium addition is the fact that the aquarist must be evaporating enough water. If the tank evaporates very little, then this reactor may be an ineffective method of calcium replacement.

The benefits of these two reactors are many. They are the only way I know of constantly maintain high levels of calcium and alkalinity while getting the added benefit of trace element additions. The real beauty of a reactor is that it doses everything slowly. When you add supplements or do water changes, there is a chance of shocking your inhabitants, or worse yet, wounding them irreparably by improperly dosing an additive. On larger aquariums, they are more cost effective over time than using conventional additives. Many aquarists have chosen to spend the money up front and enjoy the peace of mind these reactors provide.