I have been running an 8 watt Pentair UV unit on a 55 gal for a few months now. Prior to installing the UV unit, both of my tanks had chronic problems with heavy algae build up and incidents of 'ick'. Since installing the UV unit, my 6 gal tank continues to fight an ongoing battle with algae buildup and there have been two incidents of 'ick', but my 55 gal tank barely has a trace of algae and there have been zero incidents of 'ick' or any other parasite.
As I understand it, in one way of thinking UV water exposure works a bit like radioactivity. If the UV is strong enough and if the water is exposed long enough, it will kill almost anything. If the UV unit is marginally sized, and the water receives marginal exposure, free floating algae are killed off but most parasites like 'ick' remain unaffected. While the Pentair unit I chose isn't particularly powerful in terms of UV wattage, the huge module results in a very long exposure time for the circulating water. I deliberately chose to go this route vs. a smaller module with a more powerful UV wattage because ANY wattage applied to the water causes extra heating, and during the summer months it's difficult to keep my tank water temperature from rising significantly. Had I chosen a much higher wattage UV unit I might very well have needed to also add a water chiller, which would have been VERY expensive.