Though a pet fish is generally kept in largely unchanging conditions, it is not uncommon for health conditions to develop. Bacterial diseases are often very serious in nature, potentially resulting in physical deformities or death. If antibiotics are given to an infected fish, medicines chosen may include penicillin or amoxicillin. Treatments may be given topically or through ingestion.

Most conditions are highly contagious in nature, so it is important to immediately separate an infected fish from any other fish or creatures with which it shares an aquarium.

Lowered appetite and decreased movement are common signs of illness in a fish. An infected fish may also change in appearance. Eyes may appear to protrude from the fish’s head or become covered by a cloudy film. Bacterial disease may eat away at a fish’s fins, causing them to appear shredded or deformed. Red spots that develop anywhere on the fish’s body are also a sign of disease.

There are a number of common bacterial diseases that owners of fish should be aware of. These include Bacterial Ulcers, Columnaris, Dropsy, Fin Rot, Gill Disease, and Ich.

Bacterial Ulcers

An ulcer penetrates the body’s surface, destroying several layers of tissue with the potential to continue growing unless medical treatment or antibiotics intervene. Fish may develop an ulcer if exposed to bacteria in the tank, whether it comes from the water, another animal or its food.

If an ulcer is caught early, medical treatment is more likely to be successful. Whether or not multiple fish appear infected, if a fish with an ulcer shares an aquarium with other animals, all should receive similar medical attention.


Columnaris is a bacterial disease that results in white lesions on a fish’s body, particularly around its mouth. Its fins may deteriorate and white marks may appear in localised areas on the fish’s head or body.

Dirty water is the biggest cause of columnaris and it is highly contagious. Improved living conditions and antibacterial treatment can eradicate the disease, though antibiotics may be required if the bacterial disease is in advanced stages.


Dropsy is generally caused by a bacterial infection. This disease manifests itself in the form of an enlarged, swollen abdomen.

Veterinary Surgeon, London at Blue Cross UK | + posts

A London based Veterinary surgeon, Sanja is also an avid writer and pet advocate.