Those reactors will help keep a calcium intensive system stable with their calcum levels. Tanks that are heavy with corals can often have issues maintaning calcium levels due to the massive intake of calcium by the corals and the calcium reactors will help stabalize this by injecting calcium enriched water thru the system. The reactor uses co2
to change the PH
of the water within the reactor causing a faster disolving of calcium from the argonite media.
In a tank thats not having issues mantaning calcium levels or a tank whos owner does not mind daily or bidaily calcium additives its not needed.