The top of my tank is enclosed, but one of my eels jumped out one time when the arowana came up underneath him and startled him while he was eating. My 2 daughters and my wife were screaming while I got him back in the tank. It was pretty funny. One thing I have noticed with 2 big fire eels is you can't give them a hiding spot because they will fight over it. Once I removed the hollow log they were using to hide in, they never fought anymore.