If the cysts hatch, the tomites will be in the water column, and those that go through the UV filtration will be killed. Some of them will manage to find a host before going through the filter, and will hatch out another batch. The UV will reduce the odds and lower the rate of infection, but it will not stop the infection once started. If you use the UV as part of the q-tank setup, you're a step in the right direction, but not the cheapest route to take. Prevention is always more effective than treatment, and UV's do nothing to prevent fish from exposure and infection.