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The swim bladder is a sac-like organ that controls buoyancy in fish. In some fish it also aids in respiration and possibly communication. Buoyancy is controlled by filling the sac with air and floating up in the water, or releasing the air in the sac and going down in the water. By equalizing the internal density of the fish with the density in the water surrounding the fish, the fish is able to stay at a certain depth.
Some fish vibrate the swim bladder to create sounds, thought to be used in courtship. The air in the sac also amplifies sound possibly aiding in hearing or feeling.
The sac is connected to the bloodstream by an oval muscle (sphincter). Air moves from the blood to the sac and vice versa by diffusion and the use of pressure. This muscle or the sac itself may become inflamed or infected, thus causing the fish to loose control of buoyancy. If it cannot close, the air in the swim bladder escapes and the fish sinks and may lie on the bottom. If the sphincter cannot open, the air in the swim bladder cannot escape, and the fish is at the surface of the aquarium facing head down (headstanding).
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