you don't need one. They are useful to people that are REALLY into corals, specifically hard corals that require calcium for skeletal growth.
You can always use kalkwasser or calcium supplements to acheive your desired calcium level ~ 420 ppm
Basically, the reactors break down aragonite by injecting CO2
into the chamber. the CO2
causes calcium ions to be freed and a slow drip is set up to reach the desired calcium levels. Thats it in a nutshell anyway.