In the aquarium, fish jump due to the following reasons
Poor Water Quality.
Ensure that the water in the tank is periodically changed and that the tank has an efficient filter system.
Adequate oxygen levels need to be maintained by means of proper aeration.
Chased by bully fish.
Fish can jump in order to escape from a bully fish. Ensure that all fish are of a peaceful temperament. See Article Handling Bully Fish
Jumping out of the tank is one of the many dangers for aquarium fish. In the wild, fish jump to catch foods such as insects resting on overhanging branches.
Fish also jump to move to a nearby water body if the water they are living in becomes inhospitable due to low oxygen content or dries up.