A calcium reactor dissolves aragonite with carbon dioxide gas. The reactor is hooked to a CO2 cylinder and a pump to circulate tank water through the system.

A kalk reactor doesn't really "react" anything. Its basically just a mixing container. It solubolizes Kalkwasser and slowly doses it into the tank. It uses a pump to stir a Kalk/water slurry and a dosing pump to slowly add the kalk water.

Both are excellent systems but require a lot of plumbing, pumps, and in the case of the Kalk reactor you need a source of RO water to hook up to the system. Because of this, they are usually limited to larger systems that have the room to handle the equipment.