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Bacterial Diseases in Fish

Bacterial diseases are common in aquarium and pond fish. Many of the bacteria associated with disease in fish are naturally present in the aquatic environment, and so their elimination from the fish’s captive environment is virtually impossible. It also means that they are a constant threat to the fish that must rely upon a fully functioning immune system to constantly fend off invasion by the pathogens.

Common Bacterial Pathogens

Two recognized fish pathogens are Aeromonas salmonicida achromogens and those of the Pseudomonas complex, while opportunistic environmental infections include Aeromonas hydrophila, Citrobacter, and Edwardsiella. Occasionally, mixed infections of these bacteria may occur. It is thought that in some cases A. s. achromogens attaches and causes an initial lesion but is then swamped out by secondary invasion with other species.


Aeromonas Bacteria

Anything that stresses the fish or otherwise compromises its immune system may leave it open to infection. The usual suspects would include water quality, low or rapidly fluctuating temperatures, and overcrowding. Damage to the skin, which disrupts the protective mucous layer and breaches the outer layers of the skin, may allow these bacteria to attach and establish themselves on the exposed and damaged tissue. Such traumas may be physical: for instance, scale loss during spawning, or while scratching against objects. Scale loss may also be caused by large parasites, (like Argulus). Poor nutrition also plays a part. High protein levels help to produce high antibody levels and essential fatty acids aid in antibody and white cell production, while vitamins A and C are also significant. Deficient diets mean an increased susceptibility to disease.

Signs to Look For

Bacterial infections can potentially affect any ornamental fish. However, those that cause the most concern are Koi and Goldfish. It may be that these species have an increased susceptibility, or it could be that they are usually transported and kept in crowded conditions. In many establishments, goldfish seem to be treated as a commodity rather than as living organisms.

Peracute (Sudden)

Usually this category is characterized by sudden, unexpected deaths with no obvious external signs. Microscopically there is often a massive overgrowth of one particular type of bacterium. The bacteria multiply so quickly that the fish is unable to mount any sort of an immune response. Death is usually due to toxins released by the infection. The infection could be confused with water-quality problems and poisoning.

Acute (Fast onset)

These fish show classical signs of septicemia, with blood streaking and blotches (hemorrhages), especially on the skin and fins. Other symptoms are behavioral; the diseased fish lose their appetite, become sluggish, and clamp their fins. Infected individuals separate from the main group and will be found at the pond edges or aquarium corners. Again, many of these effects are a result of toxins released by the bacteria. Younger fish succumb first, and in a serious outbreak most of those affected may die. It is important to rule out parasitic infections as another possible cause of skin and fin hemorrhages. In a pond of Koi and Goldfish, another possibility would be SVC.

Chronic (Long term)

Ulcers in the body wall are probably the commonest presentation of a relatively long-standing or chronic bacterial infection. These can vary from being small, well-defined ulcers to extensive erosions of the body wall that can extend into the body cavity. Such ulcers are not just about the infection, they are a serious gap in the body’s defenses that can allow other secondary invaders, such as fungi, to colonize the wounded tissue. Also, the fish suffer from the loss of body salts and an influx of water, causing a marked dilution of body fluids.



Such fish are unable to accurately control their fluid and salt balance, and in extreme cases or where there is also significant damage to the kidneys and/or gills, than a buildup of fluid inside the body cavity (ascites) can develop. In these cases the fish have a swollen abdomen, protruding eyes (exophthalmos), and scale protrusion. This latter sign gives the characteristic “pinecone” appearance of dropsy caused by the buildup of fluid in the scale pockets.

Occasionally, bacterial infections will be localized and present as a group of raised scales. Here infection has established in the scale pockets and has caused the individual scales to become more prominent. This can progress to true ulceration. Another presentation that can be seen is long-term muscle wastage, especially with the mycobacterium infections.

Bacterial gill disease is another chronic manifestation involving bacteria. Here the bacteria are only part of the problem and gain a hold on the gills only after poor water quality has damaged these delicate tissues. Clinical signs will be respiratory distress, heavy breathing, and hanging around areas with high oxygen levels. The gills may appear swollen and have discolored patches.

When you are inspecting pond fish for ulceration, be sure to always bowl them to inspect their underbellies, as ulcers can be found lurking there and will be invisible from above.

In some cases it may be thought necessary for bacterial culture to be performed to determine the exact bacterium involved. A professional fish health expert using a sterile swab usually does this. In theory, the procedure should allow you to find out not only which specie or species of bacteria are causing the problem, but also to have it tested against appropriate antibiotics so that an appropriate treatment is started. The isolation of known secondary invaders such as Aeromonas hydrophila may not reveal the whole story, as it may have swamped out the true pathogen from the lesion.

Antibiotics are often the only truly effective treatments available, but they do have some practical drawbacks. Antibacterial proprietary machines available from retail outlets are rarely effective against bacterial disease. Mild or early infections may occasionally respond, and the tea tree-based products are worth trying in these cases. For advanced bacterial infection, I find these products of little use, however.

The presence of predisposing factors are also important, if the fish are stressed due to poor management, then no amount of medication will help them recover until their environment is correct.

To avoid bacteria becoming a problem in your pond use KoiZyme. When Koi get a wound, the "bad" bacteria consume the dead cells around the wound. They do not eat healthy cells, only the dead cells. The problem is the bacteria emit a waste product-a toxin- that kills whatever cell it comes in contact with. Therefore, as it eats a dead cell, its toxin kills the next cell so it has more food and can reproduce and on and on. Koizyme is bacterial in nature. It is safe to handle, not caustic, does not cause undue stress, and impossible to overdose. It is simply a bottle of safe bacteria. The KoiZyme bacteria also eat the dead cells but give off a waste product that is nontoxic and does not kill the next cell in line. These "good" bacteria compete with the bad bacteria. They do such a good job of it; the bad bacteria simply starve to death.


Individual fish may need injecting with an appropriate antibiotic. Groups or those that are still feeding may be given food impregnated or coated with an antibiotic. Before offering medicated food, starve the fish for 24 hours to make them hungry. If an antibiotic is available only as a table, crush the required amount until a powder is formed. Always wear gloves while doing this. Mix the antibiotic with a small amount of vegetable oil until it forms a thick paste. Mix this paste with an appropriate amount of pelleted food. Allow it to air-dry before feeding.

Remember: Prevention is the key to keeping healthy Koi.



Tricide-Neo is a patented dip to aid in the treatment of topical bacterial infections such as ulcer disease, fin rot, tail rot and mouth rot. Tricide-Neo breaks the bacteria's resistant outer layer to allow the antibiotic to stop the bacteria. Fish can be dipped every other day for one week. The fish are dipped into the bath for 3 to 7 minutes -- that's fast !!

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Resources: Common Fish Diseases by: Lance Jepson

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