A UV Sterilizer uses a germicidal bulb that kills bacteria and protozoans in the water that passes through it. It also kills plankton, so I don't think you want one on a reef tank. They are useful on fish only tanks to help control disease, as it kills the free swimming part of the disease, like ich.
A Calcium reactor is an automatic device that adds calcium, strontium and other trace elements to the water. It uses CO2 gas to lower the ph of the water in it so that the calcium based gravel in it will dissolve. The aragonite won't release the calcium at higer ph levels. They are expensive to set up, but inexpensive to run, and much better for hard corals and clams than just adding kalkwasser or calcium additives. They are starting to be more common, but still only a few reefkeepers have them. You don't need one on a fish only tank, or one without hard corals or clams.
Sonny
