If your using a compact fluorescent (screw in coil type), stop asap, like right now. These emit a dangerous short wave uvb radiation that can blind chelonia.
If your using one of these bulbs chances are your turtle has photokeratoconjunctivitis.
I'm not using any flourescent light on it, it is just in a regular plastic tank that you buy off the streets. Thanks for your suggestion though. I really appreciate it.