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Aeromonas Bacteria

Bacterial infection in a pond can be caused by numerous species of bacteria, but the most common is Aeromonas hydrophilla. This is found in nearly all ponds, but as long as your Koi are in good health, infection will generally not occur. It is only when your Koi are stressed because of temperature fluctuation, overcrowding, poor water quality and other factors which have the effect of stressing the fish that the bacteria present can develop into a problem.

If identified early, bacterial infections can be treated with the use of bactericides, but it is vital that it is caught early as advanced stages of infection may prove very difficult to treat. Flavobacterium columnare is another bacterium which may prove to be a problem. Whatever bacteria are suspected, the most vital thing to do before any treatment is applied is to try and identify the exact organism you are dealing with.


Aeromonas Bacteria

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The first course of action must be to take a swab from a Koi showing symptoms. This has to be sent away for processing. The results will give you all the information required to treat the infection, as the exact bacterium will be known, plus which anti-bacterial will be effective against it.


Symptoms of Aeromonas hydrophila include the build up of excessive mucus, and areas of reddening on the body which ultimately result in the lifting of the scales which in turn may result in the presence of ulcers. In some cases the scale lifting does not confine itself to a specific location on the body and the infected Koi may take on a pine-cone appearance. Dropsy could then develop as well as other conditions such as pop-eye.

Internal problems may also be developing along with the external symptoms. If losses do occur, internal examination may well indicate hemorrhagic septicemia around the internal organs which is identified by a reddening of these plus the presence of excess amounts of fluids and blood.

If your Koi start to develop numerous ulcers on the body and also around the head, gill cover (operculum) and mouth, it may be that new hole disease is the problem. This may be caused by a variant strain of Aeromonas salmonicida. In extreme cases the mouth of the infected Koi may simply appear to rot away. Areas of infection on the underside of the mouth are common and they are hard to notice until the disease is at an advanced stage, as this area is not easily seen by the keeper. New hole disease may cause high losses, especially in small Koi. If new hole disease does take hold in a pond, the symptoms will develop quicker than with other similar infections. Because the symptoms are so disfiguring, with loss of scales and possible scarring, even if the infected is cured, the monetary value of the treated fish can be reduced dramatically.


The best way to prevent bacterial outbreaks is to run a clean pond. It is impossible to eliminate bacteria completely from the water, so it is vital to ensure that the environment in which your Koi live is kept to the highest standard to ensure that levels of stress are kept to a minimum. This makes the chance of an outbreak far less likely.

This can be achieved by keeping stocking levels to a minimum, only purchasing new healthy Koi, and maintaining constant water temperatures without fluctuations. A good pond heating system really is essential in this respect. Other steps which can be taken to reduce the chances of a bacterial outbreak include the use of probiotics, as these may help to keep bacteria levels at a minimum.

The use of ozone may be considered, as not only will this help improve water quality and the overall pond environment, but it can also reduce bacterial levels in the water. Although not practical for every Koi pond, it is worth mentioning the use of UV sterilizers. These differ from the typical UV clarifier used by most Koi hobbyists in that they are not designed simply to stop green water, but work by exposing the pond water to a much higher concentration of UV light in order to kill waterborne pathogens.


For any Aeromonas or other bacterial infections, treatment takes the form of anti-bacterial drugs which normally have to be obtained from a vet. However, for minor infections alternative approaches, such as herbal (Melaleuca) extracts to deal with small bacterial ulcers, may be considered along with the possible use of off the shelf proprietary bactericides which will be available from your local Koi dealer. When deciding which anti-bacterial to use, you must refer back to the results from the swab to see what is going to be effective. Generally for larger Koi it will be necessary for each Koi showing symptoms to be administered an antibiotic injection but advice should be sought from your vet or local Koi specialist before any treatment is given. It is usually easier to do this if the Koi sedated. At the same time it is important that any secondary infections like fungus be treated, along with any ulceration, by the application of a topical treatment such as malachite green and propolis. When topically treating your Koi, it may be necessary to remove lifted scales to help deal with the infection.

If a large number of smaller Koi are infected, it may be impossible to inject them all. In this case you can use anti-bacterials mixed with the feed.

You can also use Chloramine T at the minimum dose of 1g per 1000 liters (220 gallons). KoiZyme is another solution for bacteria infections, this is simply a bottle of safe bacteria. KoiZyme is safe to handle, not caustic, does not cause undue stress, and impossible to overdose.


You are here: Home > Koi Health & Wellness Center > Koi Diseases > Aeromonas Bacteria

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