Ok CA reactors add both calcium and alkalinity to the tank in equal amounts. this is important becuase they are also consumed by corals in the same amount. Other additives like CaCl and NaCO3 will add NaCl over time and the salinity will rise. With a calcium reactor we are able to add only Ca and CO3 (alk) without effecting salinity. It is also cheaper over time to buy CaCO3 and not the other compounds. Another plus to Ca reactors is that the media typically also contains trace elements like strontium, magnesium, and others.
The basic concept is to take calcium carbonate media and dissolve it in the tank. This media is often made up of old coral skeletons for example. So you can think of it as dissolving old coral to provide those elements back into your water for your new corals to grow. Kind of ironic. The problem is that CaCO3 will not dissolve at the pH of seawater. You need a mild acid. So what you do is to provide a reaction chamber that has your media and you pump carbon dioxide into it. The CO2 will make carbonic acid and the pH will drop. Now the CaCO3 will begin to dissolve and release Ca, CO3 (alk), and other trace elements into the tank.
Pros: doses Ca, Alk, and trace. Cheaper long term solution than adding each compound separetly. Has no effect on salinity. Not very dangerous (as opposed to Kalkwasser CaOH).
Cons: Large start up cost. Can be a pain to fine tune. Requires lots of equipment so you need to monitor it all. Can effect your tanks pH if you don't monitor flowrates and effluent pH.
Overall, I would highly recommend calcium reactors for larger tanks and totally not recommend them for smaller tanks.
FB