All three eels I have kept have been in open topped tanks. The first couple days seem to be when they will jump, if they get established then they seem to do fine. A good size eel (24") seems to be able to hang out on the ground for about 8 hours or more. It happened to one when he first got put into the tank. He looked like eel jerky, but when I put him back in he came around after a couple days.