You mean calcium reactor? You run carbon in filter socks or through canister filters most likely. If it is a canister filter such as a fluval, they come in packages made for them by fluval you can buy or if you get a filter sock, just get some carbon such as the marineland carbon in the big container and fill the filter sock somewhat and stick it in your sump.
Calcium reactors are a install it and leave it type of thing. If you get big into clams and SPS corals that are all over your tank, the calcium consumption will be a lot and a calcium reactor will keep it stable. With this, you use calcium media in the reactor.
You do not need it as you can dose the 2 part manually whereas a reactor would be much easier.