nice idea...but i'm afraid it wouldnt work....
UV bulbs, are VERY strong, you can do serious damage if you look at them. Even with this huge power, the water must travel very close to the bulb, within about 1 or 2 cm to have any major impact on parasites etc, hence the reason pond UVs (clarifiers, not sterilizers)don't work.
So a weak blacklight suspended 5" over the water would do nothing.
It would look cool, but I am sure I have read about negative effects on inhabitants.
If anyone knows otherwise....feel free to chime in
