The swim bladder is a small organ inside the fish that can fill up with air or deflate, allowing the fish to move around
in the water column more easily. The swim bladder is quite prone to infection, which can cause bloating, disorientated swimming,
and involuntary floating or sinking. Alternatively, the gut of fish may have trapped air, caused by constipation or by an
infection.
Gut and swim bladder problems are notoriously common in fancy goldfish, which have been over-bred to create their unusual
shapes. In some cases, this has adversely affected the structure of the swim bladder.