How do fish maintain buoyancy in the water?

Fish maintain buoyancy in the water by using an organ called a swim bladder.  The swim bladder stores oxygen gas which helps the fish maintain a level depth.  The swim bladder is located in the center of the fish’s body and acts like a balloon.  When the fish wants to go up, they take more oxygen into their swim bladder and when they want to go down they expel oxygen from their swim bladder.