I have a Coralife 9v UV sterilizer piped into the outflow pipe of my canister. I don't know that it is absolutely necessary for clear water but to me every little bit helps. UV will not prevent algae that attaches to things. It only treats things suspended in the water. It will not allow you to run your lights longer either. The ones that fit in the tank and are movable are only good for green water, nothing else really. The bigger units usually have a chart in their paperwork that shows at what flow it will sterilize different things. Mine shows at full flow it will treat green water algae, at half flow(of my filter) it will get bacteria and at real slow <25% flow it will kill most pathogens. I've found that with doing regular water changes, clean filters and balanced light and ferts its more of a luxury than a necessity. I also found you can put it on a timer and run it only eight hours a day just as good as running all the time. This way your bulb will last much longer. Hope this helps. OS.