Every fish in your aquarium is capable of jumping out. The ability or tendency to jump isn’t based on a fish’s shape or personality alone. Sure, some fish species are known more for jumping than others, but every fish is capable. Fish that do jump usually do so because they are startled by someone approaching the aquarium, are trying to escape a pursuing fish, or are just trying to get out of a stressful situation. To keep jumping fish in the aquarium, install a mesh screen over the top of the tank. Many DIY projects are available on the net.