Introduction
Buoyancy disorders are common in goldfish, often having a sudden onset and present floating at the surface or lying on the bottom. In many cases, there were no external lesions and no consistent environmental factors could be identified. In fish, buoyancy is controlled by the amount and distribution of gas within the body. This is primarily enclosed within a gas-filled buoyancy organ, the swim bladder. In some fish, physostomes, there is a patent duct that connects the swim bladder to the anterior oesophagus, which permits air to be swallowed and forced into the swim bladder. In other fish, physoclists, there is no patent connection and the swim bladder is inflated by the release of gas from arterial blood by a vascular rete in the wall of the swim bladder: this also occurs in some physostomes.
When a fish is totally immersed in water in water it displaces an amount of water equal to its body volume. If the weight of the displaced water is greater than the weight of the fish, the fish is said to be positively buoyant and will tend to float. If the weight of the water is less than that of the fish, the fish is said to be negatively buoyant and will tend to sink. If the weight of the displaced water equals that of the fish, the fish is said to be neutrally buoyant. So, underwater, the weight of an animal is counteracted by its buoyancy (Mandy Dyson 2003).