I dont think you can put them into a sump ... if you are talking about a CaCo3 reactor (that requires a Co2 bottle and regulators) they are very easy to make .. for a fraction of the cost ... I posted a thread about my DIY CaCo3 reactor Over HERE .. I am quite surprised how well my tank has taken to this reactor ....
depends on what you're keeping. If you're keeping SPS or any other Ca loving corals, they're a GREAT way to keep your levels in check.
Ca reactors and Kalk reactors (aka Nilsen reactors) both will do a good job of keeping your Ca and Alk in line. Ca reactor has a little advantage over the Kalk reactors though. The MAIN differences are:
1. Ca reactor maintenance is much easier (actually isn't much. Change the media every 6 months, and if you have a pH controller, calibrate it from time to time. That's it).
2. Ca reactor not only keeps your Ca and Alk in line, but also add *some* trace elements back into the water.
Ca reactors don't really go IN the sump, but usually sit next to it, so you can still hide them from view. The CO2 canister is a little different, but you can still put these somewhere else out of sight.