Leaf fish tend to be sedentary. Their normal behavior is to drift along in the current until they are close enough to any small fish, which they then eat. They have an extendable mouth peice which shoots out to catch their prey. Anything about 30% their size or smaller is fair game, maybe a bit larger. They are a neat fish, but won't appreciate a tank with a lot of current, will want some surface plants/debris (floating wood), and are not good community fish.
I have heard of people having big problems getting them to eat. I wouldn't recommend them to an unexperienced aquariast.