How can I get rid of gnats in a terrarium?

I have a long 10 gallon terrarium (turtle cage) and I have problems with gnats. I change the dirt frequently, but that does not help long at all--maybe 2-3 days. Any other suggestions or safe pesticides?

I didn't see the part about the turtles -lol- Nevermind...pfft..

Mix a very little bit of dish soap in some water. Spray in on the plants. It will kill the bugs and not harm the plants.

1. Soil is not a safe substrate for turtles- it holds too many microorganisms and cannot be cleaned well. Instead, try pea-sized river gravel, corrugated cardboard, rabbit chow, etc.2. If you feel you need to use soil, bake it before using it to sterilize it, and replace it every few onths with more sterilized soil.3. Keep the fruit and food fresh- don't let it sit longer than 30 minutes or so. (I bet they are really fruit flies!)