Just Keep Swimming

I was watching the movie Finding Nemo when I became mindful of some of the fish's ability to move up and down the water with ease.  The reason they can do this is because they have a swim bladder that enables them to decrease or increase their volume without altering their mass. To move upward, they fill their bladder with air acquired through their gills. This increases their volume and decreases their density, enabling them to ascend with ease.  To descend, they let the air out of the bladder. When they are swimming at a constant level, they fill their bladder to the point where the fish displaces the volume of water that weighs what the fish weighs. This way, the gravity force pulling the fish down and the buoyancy pushing the fish up cancel out and the fish can swim at the same level.