Pseudomoniasis in Fish

Causes, Treatment, and Prevention

Koi with side ulcer possibly from pseudomoniasis

Jessie Sanders

There are many bacteria species present in your fish tank, including Pseudomonas. Most aquarium bacteria are benign, and some are even beneficial for your fish, including those residing in your biological filtration system. Other bacteria, however, wait until your fish's defenses are lowered so they can take advantage of a weak immune system, leading to symptoms such as ragged fins, body ulcers, and loss of appetite.

It is rare for a bacteria species to cause serious problems in your fish on their own. Usually, they cause a secondary infection after the fish has been weakened by a traumatic injury, transportation stress, or a parasite infestation. However, some species of Pseudomonas are able to cause debilitating disease in otherwise perfectly healthy fish, and can even lead to the fish's death.

What Is Pseudomoniasis?

Pseudomoniasis is the name of a bacterial infection caused by a Pseudomonas species bacteria. It is sometimes called Pseudomonas infection. There are many different species of Pseudomonas in the aquatic environment, and the majority are not going to cause your fish any issues. Most aquatic bacteria live on the detritus that collects on the bottom of the aquarium or on the surface of plants and even the aquarium glass. They do not affect the fish unless some other stressor or lesion first weakens the fish's immune system.

In pseudomoniasis, the fish's weakened immune system permits the bacteria to switch from benign into a harmful source of fish disease.

Symptoms of Pseudomoniasis in Fish

Pseudomoniasis in fish resembles many other bacterial infections. These generalized signs are not specific to a Pseudomonas infection, but are commonly seen with any bacterial fish disease.


  • Fin erosion and redness at the base of the fins
  • Ulceration or red spots on the skin
  • Lethargy
  • Decreased appetite
  • Sudden death

Your first indication that something is wrong will likely be spotting red sores on your fish's body, or ragged fins. Often, the sick fish will keep the fins close to its body and may show signs of lethargy, which a fish displays by not moving much, drifting in the lower levels of the aquarium, and loss of appetite. You may notice your fish opening and closing its mouth rapidly. At times, however, the infection will kill the fish before obvious symptoms develop.

Causes of Pseudomoniasis

Pseudomonas species are present in almost all aquatic environments but normally only become pathogenic, meaning capable of causing illness, when a fish becomes debilitated. Once a fish's defenses have been lowered, many pathogens, including Pseudomonas bacteria, can spread and flourish. However, there are particular Pseudomonas species that can cause disease with little to no prior decrease in immune function.

One such species is Pseudomonas aeruginosa, which is a part of the normal fish microbiota. This bacteria species has become highly opportunistic and pathogenic, however, meaning that it is able to infect and sicken even fish that were otherwise healthy. Pseudomonas aeruginosa can cause serious fish illnesses including hemorrhagic septicemia, gill necrosis, abdominal distension, and internal organ failure.

If a fish's immune systems are continually weakened, or the bacteria are too plentiful, your fish will not recover without treatment.

Diagnosing Pseudomoniasis in Fish

Most fish owners don't take their sick fish to the veterinarian for diagnosis or treatment, though there are vets who specialize in aquatic pets.

When it comes to pseudomoniasis, the best diagnostic data is obtained from euthanizing a moribund fish (a fish showing severe clinical signs and close to death), and submitting the body to a veterinary lab for bacterial examination. While euthanizing a fish is never pleasant, it can be done quite humanely by immersing the sick fish into a suitably sized container filled with a 1:4 ratio of vodka (or a similar alcohol) and water. For example, if you use 8 ounces of water, you'll add 2 ounces of vodka. The fish will succumb to alcohol poisoning within minutes.

Tissues collected from the kidneys and spleen provide a great deal of information about what bacteria species is the true causative agent of disease. Then, if there are other fish in the aquarium also suffering from similar symptoms, it will make it easier to treat them with the appropriate antibiotics.

Additional diagnostics include sampling open wounds for bacterial culture and antibiotic sensitivity testing. However, external wounds are a breeding ground for many different bacteria species, so the results may not be too specific as to the actual cause of the disease.


The best way to treat Pseudomonas infections is to identify the primary stressful event that led to the fish's lowered immune function. Common fish stressors include poor water quality, inadequate or incorrect nutrition, or aggression or bullying issues. Possible solutions, depending on the primary issue, may include better aquarium or pond maintenance, removing fish from the environment, purchasing new food, and adding decor items or other environmental improvements.

Work with your aquatic veterinarian to get advice on how to improve your fish's surroundings to limit stress. If the stressor is not removed, treatments such as correcting poor water quality or antibiotic therapy will only provide temporary relief.

Your veterinarian can swab the sores on the infected fish's body and send the samples to a lab where they can be cultured to look for the Pseudomonas bacteria. Based on the bacteriology results, an antibiotic sensitivity test can be performed to determine the most effective antibiotic for that specific bacteria. This is critical for many Pseudomonas species since there is known resistance to multiple antibiotics.

Once a suitable antibiotic has been identified, your veterinarian will determine the best treatment method. Depending on the size, status, and total number of fish affected, the drug may be given as a bath, in food, or directly injected into the fish.

Over the counter antibiotic-like products are usually not effective against Pseudomonas species bacteria.

Prognosis for Fish With Pseudomoniasis

If your fish is only mildly ill and receives treatment for the infection promptly, it is likely to survive. However, fish that are severely ill, or fish not treated with the appropriate antibiotic, are likely to succumb to the Pseudomonas infection.

How to Prevent Pseudomoniasis

The best method to prevent the spread of Pseudomonas bacteria in your aquarium is to adhere to strict quarantine protocols. This way, if a fish that is stressed out from capture, transport, and the new environment starts to show clinical signs, it will not spread the disease to the rest of your fish. And maintaining a clean and well-balanced aquarium water system, avoiding overcrowding the tank with too many fish, and feeding a quality diet all help keep your fish healthy enough to fight off infections before they begin.

Is Pseudomoniasis Contagious to Humans?

Pseudomoniasis is not typically transmissible to humans. Immunocompromised individuals, children, and the elderly should limit their contact with aquarium water, just to be on the safe side, however. The most common fish bacteria with the potential to infect people is fish Mycobacterium, which can cause severe skin lesions in humans.

Article Sources
The Spruce Pets uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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