put a lid on it !
I just mentioned this to my mate who keeps but he said he's never experianced them jumping !
Many fish are capable of jumping, my green spotted puffer use to do that
when the lights turned off. seemed to scare him then darted off and jumped out of my tank with the lid on !! , managed to jump out where the hood is cut out for the cables to go through but lucky for me i was in the same room and could hear a flapping !!!
Resorted to using a bit of tape ( not sticky of course !! ) and changed the loud noise making timer to an electric one.
or try using a clear glass / plastic top if your tank is open.
Plants would be no good as they wont survive
and lowering the water level is usually a no no to look at.