Have you ever thought how do fish move up and down in water? Which organ helps them complete this task? I know common people don’t use their brain that much level. Fish have an organ called air bladder or swim bladder which helps change buoyancy to make them able to float in a three-dimensional environment – up and down, forward and backward and to the sides.
How buoyancy functions
In order to float, fish must shift some amount of water which may be less or equal to its own body mass. At the surface level, it is easy to relocate the water, but inside the water it becomes very tough because at that level the density of water changes depending on depths and different water currents. The organ swim bladder with full of air makes fish able to move upwards in the water while moving downwards it releases air. Unfortunately, air bladder is not available in all of them. That’s why fish with no air bladder can only move forward along with pectoral fins so that they could float continuously.
Source: https://animals.mom.me/fish-maintain-buoyancy-5539.html