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The Most Common Bacterial Infections In Aquarium Fish

Bacterial infections in aquarium fish can occur when water quality is low. Learn about the various types of bacterial infections and effective methods for prevention.


Bacterial infections are fairly common among freshwater aquarium fish, especially in tanks where the water quality is low. Low water quality may be caused by overfeeding, inadequate filtration or a number of other factors. When the water quality in your tank declines, your fish are likely to become stressed, which increases their susceptibility to disease. When this happens, fish that are exposed to bacteria are much more likely to contract an infection. Once one fish in your tank contracts a bacterial infection it may not be long before it spreads to your other tank inhabitants. For this reason, it is wise to cultivate a basic understanding of common bacterial infections in aquarium fish so you will be equipped to recognize the symptoms and make a quick diagnosis.

Common Bacterial Infections

disease xs 20243547Fin Rot – This disease typically manifests in the form of a bacterial infection affecting the tail and fins of infected fish. Though it is usually brought on by poor tank conditions, fin nipping and bullying can greatly increase the chances of a fish contracting this disease. Over time, this disease results in the fraying and total breakdown of the tail and fins. At first the fins may lose color but, if the condition isn’t treated, the entire fin could be destroyed. Antibiotics are typically effective in treating this condition and they are best administered after the fish has been moved to a hospital tank. A hospital tank will provide the infected fish plenty of time and space to rest and it will also give the fins time to regrow.

Tuberculosis – This deadly disease can affect all species of fish and it is the only aquarium fish disease that can be passed to humans. Fish that have tuberculosis may not show any symptoms at first but may eventually exhibit a loss of appetite, discoloration, emaciation and visible deformities. Once Myobacterium piscium, the bacteria responsible for tuberculosis in fish, has entered the tank it can be very difficult to eradicate. Because there is no known cure, this disease typically results in the death of the affected fish. Although this disease cannot be cured, treatments such as Kanamycin and Vitamin B-6 have been known to improve symptoms.

diseaseMouth Fungus – Caused by the bacterium Chondrococcus columnaris, mouth fungus is also known by the names Columnaris and Cotton Mouth Disease. This condition may first present in the form of a white or gray line on the lips of fish but will eventually grow and spread into a white, cottony growth. The fungus growing on the mouth of infected fish produces toxins which, in combination with the growth itself, result in an inability to eat. Infected fish may become emaciated and can die if the infection is not treated quickly enough. Penicillin is the recommended treatment for this condition though other antibiotics such as Chloromycetin, Maracyn and Kanacyn have been known to be effective.

Dropsy – Though it is caused by bacteria, dropsy is not so much a disease in itself but a symptom of a larger problem like kidney failure. Dropsy is caused by a bacterial infection in the kidneys of affected fish resulting in the build-up of fluids. This build-up of fluids can cause the belly of the infected fish to bloat and the scales to protrude from the body. In this condition, fish are likely to have difficulty swimming and may decline food. If the fish is still eating, however, it can be treated by adding a 1% antibiotic to the food.

PestRed  – This disease is named after the bloody red streaks that form on the bodies of infected fish. Though this bacterial infection is typically internal, it does produce external symptoms like red streaks which, if untreated, can lead to ulcerations and fin rot. In extreme cases, the fins and tails of infected fish have rotted away entirely. In order to treat this condition, the first step is to disinfect the tank with a 0.2% solution of monacrin (monoaminoacridine) or acriflavine (trypaflavine) at a rate of 1 ml per liter of aquarium water. The next step in treatment is to add an antibiotic like chloromycetin or tetracycline to the food given to infected fish.

Vibriosis – This disease is caused by a number of strains of infectious bacterium belonging to the genus Vibrio. Vibriosis is most common in marine and brackish aquariums but it has been known to affect tropical species of freshwater fish as well. Fish may come into contact with the bacteria through open sores or by feeding on dead fish that have been infected with the disease. Common symptoms of vibriosis include lethargy, rapid breathing, loss of appetite and hemorrhaging. Hemorrhaging may begin with red streaks under the skin which develop into swollen lesions and eventually turn into ulcers. This disease tends to progress very rapidly and can become fatal before a diagnosis is even made. The best treatment for this condition is oral antibiotics like Kanamycin, chloramphenicol and furazolidone.

Tips for Avoiding Bacterial Infections

filter2 fluval-406 1Because bacterial infections are common in tanks affected by poor water quality, the key to preventing these infections is to keep your tank clean. Avoid overfeeding your fish in order to prevent excessive waste build-up and ensure that your tank is outfitted with an adequate filtration system to help remove solid as well as dissolved wastes from the tank. Regular water changes are another essential part of keeping your aquarium clean. Unless you change 10% to 25% of the water in your tank on a weekly basis, toxins and waste could build up in your tank water and bring down the water quality. Not only does poor water quality provide an ideal environment for bacteria to grow but it may also put stress on your fish, thus making them more susceptible to disease.

Perhaps the best way to prevent bacterial infections – and all kinds of aquarium fish disease – is to avoid introducing sick fish into your aquarium. Before you purchase any aquarium fish, observe it in the store tank for signs of illness. The fish should be active and display healthy coloration – it should not be lethargic or erratic in its movements; it should not have cloudy eyes or visible sores or deformities; and its fins should appear healthy and not clamped or frayed. Once you bring your fish home, keep them in a quarantine tank for at least two weeks. If after the two week period has passed and the fish have shown no signs of illness it may be safe to introduce them into your main tank.

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