Gnats keep getting into my hermit crab tank, will they be okay?

Gnats are getting into my tank, and I fear that they will kill my hermit crabs. I currently have 3 crabs (Ms. Hermie, Cloudy, and Tiger). Whenever I open the lid to the tank to refill the food or water, the gnats get in. They climb all over my hermit crabs, and I can't do anything about them. I am VERY worried about my little hermies. :( If the gnats will hurt my crabs, what can I do to save them? PLEASE HELP!!