I think on a qt tank uv will cut down on bacteria plankton and possibly velvet spores for a time , if it's big enough but it won't irradicate them except in single pass operations,ie the water doesn't go back to infested water . Single pass is how they are commonly used in fisheries . In a recirculating system like an aquarium those organisms not killed by passing through uv just proliferate to make up the losses.
Killig bactria plankton in a reeftank might not be a good thing to do as they are a bottom floor part of the food chain , bind up excess metals and are skimmable along with the nutrients they metabolize.