How do you keep your snake tank humid?

No, I'm told they can get fungal infections under their scales, but it's never happened to my snake and I live in the troppics. Mind you my snake likes to hang in his branches rather than lie in the water. I think it only realy happens when your snake lies in water too much. Some people I know have to give their snake a very small water dish to stop this happening. I think snakes also like to bath in their water before shedding their skin.