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I keep my turtle tank on the windowsill in my dorm room. Despite the fact that it encourages aglae growth, it doesn't seem to be doing any harm to my turtle. She and the feeder fish seem to enjoy the natural light. However, I've recently realized that it isn't doing them much good in terms of providing UVB light.

If I provide a UVB lighting source, is it possible that its light combined with whatever UVB that comes through the window might be too much UVB light, resulting in some kind of turtle sunburn or something?

Contrary to what you believe the turtle is getting no benefit whatsoever from the sunlight coming in through the window. Just as people can't get sunburn through glass.. ie a car window.. neither can a turtle which only shows that they don't get any use from the sunlight that way. You need to have a special UVB bulb over the basking area for your turtle to get the healthy benefits of the UVB rays. You could also take your turtle outside in full sunlight. The outdoor temps should be in the 70s and sunny.

There is a danger to having your turtle's tank by a window, in that it could overheat the water and kill your turtle.

You may wish to read this info.

http://www.redearslider.com/index_lighting_basking.html

I have read that actually, but thank you for the link.

I constantly monitor the basking and water temperature, and to date they have yet to rise to dangerous levels. I keep the water at 80 degrees and the ambient air in the tank at 90 degrees. There is substantial tree cover, and especially now in the winter months when sunlight is less direct it does not provide much heat at all. I have yet to have the turtle during the summer months, so I might have to move the tank in June or July. Anyhoo...

My question was more in regard to whether it is possible for a turtle to get too much UVB light at all, by whatever means. I am going to purchase a full-spectrum fluorescent bulb soon. Some sources say that though window-filtered sunlight alone is not suitable as a UVB source for turtles, sometimes very thin windows without exterior screens let a very small amount of UVB light through. I was worried that 12 hours of UVB light from the new bulb combined with any UVB light that may be sneaking through the window might be too much for the turtle. Is it even possible for a turtle to get sunburn, or sun poisoning, or things of that nature?

Again, thank you for the feedback.

I don't think it's possible to get "too much" uvb, as even with a uvb light and if there is anything actually getting through the window, it won't be more than about 12 hours a day.

Also, be careful about "full spectrum" lights. These usually don't actually provide any significant uvb light. You need to find a light that says specifically the amount of uvb light. The most common strengths are 5.0 (5% uvb light) 7.0 (7%) and 10.0 (10%). I think the brand that most users on this site use is Zoo Med Reptisun.

You can get them in two different shapes, This type, which fits in most aquarium hoods, or this type which fits in a standard light socket.

Sounds good. . . thanks!