A uv sterilizer may help with the free-swimming stage of ich a little; however in no way will it rid your tank of, or prevent ich. They are good for algae in the water column or green water, and they help with bad bacteria and any parasites that have a free-swimming stage in their life cycle; however as said previously they won't prevent or rid your tank of any disease or parasite entirely. They do help, but I wouldn't get one thinking you won't have a ich problem again or that you won't have to treat your fish. The only way to prevent ich from entering into your main tank is by making sure not to add it. Quarantine everything before adding it to your main tank, including live rock, sand, fish, inverts, coral, etc. All these things can contain the parasite. Even though inverts, corals, rock, and sand aren't directly infected by the parasite they can still contain it and transport it.