Calcium reactors are an excellent way to replace the need to dose a reef aquarium. Calcium reactors dissolve dead coral and slowly drip it into your tank. Dead coral contains everything that you would typically need to supplement into your reef tank.
Calcium reactors simplify issues around dosing significantly, but the cost can be prohibitive for some. The reactor itself is not terribly expensive but requires a CO2 canister, tuning the release of CO2 into the reactor, and using a doser to drip the melted coral into the tank water. It also requires regularly replacing the canister and some other additional maintenance related to the extra hardware.
Overall, calcium reactors are an excellent alternative to dosing due to their ability to maintain reef tank parameters very steadily. And consistent parameters are more critical than parameters that are simply “within spec.”