Not being an aquatic frog, there's no reason they should spend time in the bowl. I personally never use a water bowl for tree frogs. It's more of a safety measure for people new to tree frogs or who are going on vacation and are afraid the level of humidity will drop too low for the frogs. Believe it or not, since they're not spending much time in there, it's an indication that you've got a good level of humidity in the terrarium.