From The Aquarium Wiki
What is it
Balloon is a generic term used to describe various fish species (usually freshwater species) which through careful selection to enhance and stabilise a genetic deformity that makes the overall fish body much shorter and rounder in shape, hence the name.
This is done as the fish shape is pleasing to people. Side effects are that the animal will be slower in swimming and tend to have swim bladder problems (infections and less ability to adjust its height in the water column). It often tends to have digestive problems as the stomach and intestines are distorted within the body making it more difficult for it to digest normal foods. These types of fish tend to need a special gel like diet to aid digestion as they get easily constipated.
Some balloon fish may have difficulty in mating naturally and often only breed by human intervention.
- The aquarium environment needs to be designed to allow the fish ample room to turn and to remove any sharp objects it may bang against due to its lack of manoeuvrability.