The only reason I say that is because I have seen them jump at least a clear 6 inches out of the water for food. Also they like to jump if they see a fly or something over the tank. With an open too tank and a rather daft fish they could well end up on the floor.
Have u noticed that for there size and reputation they are I fact quoted placid fish with odd behaviour. My Oscar is the biggest fish in the tank. He has no problem with attacking me but then loves the other fish and showed no aggression at all. Strange fish but very rewarding.