Many fish jump naturally, such as Bettas, Endlers, Swordtails and Hatchetfish.
Generally, all aquariums should have a cover, because various things like a pH swing, sudden movements near the aquarium or even the lights coming on or going off can make any fish jump.
Make sure that the cover has NO holes in it, especially with a known jumper. Fish have an uncanny ability to jump through any hole in the cover. (except ventilation holes).
