You should seek the advice of your veterinarian if your pet is ill as only he or she can correctly advise on the diagnosis and recommend the treatment that is most appropriate for your pet.

Even though fish are relatively simple organisms control of swimming is quite a complex thing. Many disorders can lead to abnormal swimming behaviour - even swimming upside down 

If you have ever owned goldfish or other ornamental fish, you may well have seen individuals that seem to lose control of their swimming - they may swim on their sides or upside down, swim in spirals or float in the water. Oftentimes they float or swim near the surface of the water - other times they cannot seem to get up from the bottom of the tank/pond.

Normal swimming requires both the nervous system and the muscular system to be intact and working properly. Buoyancy is controlled through a special organ called the swim bladder.

Any disease which affects the nervous system , muscular system or swim bladder can cause abnormal swimming behaviour. Any debilitating disease that causes weakness will also lead to inability to swim properly, and so it isn't surprising that fish which are terminally ill will be seen swimming abnormally - even up-side down.

Because there are so many potential causes diagnosis requires a full history and examination of :

• The environmental conditions - temperature, oxygenation of water etc
• The presence of toxins in the water or food
• The presence of infectious diseases - bacteria or viruses
• The presence of parasitic infections

Disease that have been documented to affect swimming include :

• Sleeping sickness. Caused by blood flagellates (protozoans) - egs cryptobia (which affects cyprinids including carp, goldfish and tench) and trypanosoma. Fish contract the disease having been bitten by leeches. There is no specific treatment for the disease, so remove infected fish and leeches from the aquarium.
• Costia - a protozoan now called Ichthyobodo necator - affects coldwater fish and freshwater tropical fish kept below 25⁰ C . They sometimes scrape themselves on objects as well, and they develop a white film over their body.
• Fish Tuberculosis - (caused by mycobacterium and nocardia spp). TB is relatively common and affected fish can show a variety of different signs but loss of condition is very common. This disease is a potential zoonosis. Remove dead and dying fish to prevent cannibalism and transfer of the disease.
• Hole-in-the-head disease (caused by the parasite Hexamita spp). Most commonly affects Discus fish .
• Ichthyosporidium - a fungal disease that affects freshwater and marine fish. It causes weight loss and skin defects. Treatment is unlikely to be successful in advanced cases.
• Neon disease First reported in Neon tetras - but is seen in other fish as well, including other Characins and some Cyprinids (eg Zebra Danios). Caused by a sporozoan: Pleistophora spp. There is no specific cure.
• Swim bladder trouble - causes loss of balance, and fish may not be able to get down from the top of the water , or they may not be able to get up from the floor of the tank

Diagnosis can only be confirmed by taking samples for laboratory testing, or by performing a post-mortem examination. Always remove affected fish from your aquarium and seek veterinary advice.