UV sterilizers if used under the proper flow, will kill bacteria, protozoa, algae spores, micro-organisms, arguably fish diseases as they pass through. While the wattage is debated, particuarly if you just want nuisance algae or disease control. I'd probally look at a 16/25-40w range of sterilizers. You're looking at between $110 to $240 depending on the wattage and brand.