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Topical treatment of bacterial infections.


Ulcers can occur at any time of the year through injury caused by protruding objects in the pond, feeding frenzies, flashing while trying to get rid of parasites, spawning, netting, jumping out of the pond, “bag rub” during transport, parasites, bacterial infections and many more reasons depending on the specific pond.

Mostly small injuries will go unnoticed because a fish struggle continuously to prevent water from entering the body through osmosis. In water temperatures of 18 degrees Celsius and higher, a small wound caused by injury is therefore sealed by a thin membrane of skin and mucus as soon as possible after injury, and complete healing will occur without complications. Opportunistic bacteria are a natural occurrence in any pond. A comprehensive article on bacterial infections can be viewed here, and it is therefore sufficient to say that should opportunistic bacteria get access to the fish through the wound, the healing process will not be as smooth and intervention by the koi keeper may be necessary.

The intervention mentioned can take on various treatments but the main aim should be to give the fish a decent chance to overcome the infection and to start the healing process. This is done by sterilising the wound as best as possible and sealing the wound against osmosis of water into the body. The most important thing is to treat as soon as possible, to prevent the bacteria from doing too much damage and to get the recovery process under way.

This treatment can be effected with various medications that one can normally find in a first aid box and will be described later. The off the shelf treatment of choice that is available to hobbyists in South Africa are Woundmed from Bao Bio, and Pro-cure from Koi Water Barn. There may be others available that is equally effective in other countries. The main ingredient should however be a disinfectant and a sealer.


The first step is to be well prepared for the treatment. I normally set up two well aerated containers filled with pond water. The first container contains the anaesthetic while the second container will be used for recovery after treatment. Also a convenient work surface is necessary to treat the fish while under anaesthetic. On this surface I normally place a piece of high density foam and cover it with a wet towel. Fish are not used to gravity and a soft pad will prevent bruises if a fish struggle during the treatment.


Catch the fish and place it into the anaesthetic and utilise the time it takes the fish to become subdued productively by examining the fish closely and to decide on the treatment to follow.


Turn the fish upside down and inspect it for lesions not seen from above. Run a finger over the wound and determine if there are any raised scales, damaged scales or dead scales. Damaged scales will appear to be half eaten away and dead scales feels rough to the touch, like sand paper.


When the fish do not react when lifted from the container, wrap it in a soaked towel or something similar and just leave the area to be treated exposed. The reason for this wet towel is that the fish will remain moist and it will also be more convenient to restrain the fish, should it starts struggling.


At this stage it will be necessary to remove necrotic tissue, loose skin and any damaged or dead scales. The photo below shows the dammage done by bacteria to one of the scales.


Remove the scales by firmly gripping it with a strong pair if tweezers and pull firmly towards the tail section. If there are any scales with red inflammation underneath it, it should also be removed. This will enable you to treat the infection that is obviously below the scale and protected from treatment. If there are any raised scales next to the wound, water is trapped there and may harbour bacteria. It is better to drain the trapped water from underneath them by gently dabbing and squeezing them with a paper towel towards the direction of the tail, until all the fluid is released. If the treatment is done properly the first time, the procedure will not have to be repeated. You have gone through all the trouble to anaesthetise the fish, therefore you may just as well treat it properly. The biggest mistake made by koi keepers is that they become anxious and wish to get the fish back into the recovery water as soon as possible.


Use a paper towel to dab the wound dry. Make sure the wound is as dry as possible and apply the medication, by either spraying it onto the wound or by painting it on with a brush or cotton bud.


Medication like Woundmed and Propolis has handy spray nozzles to apply the medication. Spray or paint a thin layer over the wound and blow it dry. Here we use a hand operated blower that one normaly use to kick-start a fading campfire


When dry to the touch, apply a second layer and again use a blower to dry the medication properly. It is best to make sure the medication is completely dry to ensure that it will remain on the wound as long as possible.


If necessary the fish can be injected to complete the treatment.

Photos by Pieter Odendaal (Jnr)

The fish can now be placed into the recovery bowl for a few minutes and then returned to the pond. (The air stone has been removed to take the photo)


Loolilocks manufacture a wound powder that is applied directly onto the ulcer and when the fish is released and the powder comes into contact with water, it turns into a dark blue gel that sticks like crazy. In this instance the wound is cleaned like described above. The powder is then sprinkled over the wound and the fish is released. The powder also contains an antibiotic. This powder is really the medication to use when it is not possible to sedate the fish.

Bao Bio also produces another medication called Finmed. This treatment consists of two containers, a rinse and a sealant. It is specific for application on damaged fins or problems caused by fin rot.


In order for the fish to recover, water conditions should be impeccable. If it is a major lesion that has been treated, the fish will benefit from the addition of salt to the pond by reducing the osmotic gradient between the pond water and Koi tissue. As a rule of thumb, you can add three kilogram of coarse salt to the pond or quarantine pond for every thousand litres of pond water.

Keep an eye on the fish and after three days inspect the wound again. Sometimes a second or third treatment may be necessary. This time however, just dab the wound dry and try not to damage the thin membrane that may have started to form over the wound by wiping it dry, or by applying harsh antiseptics. Apply the medication and make sure it is dry before releasing the fish again.


removing_daed_scale_2nd_treatment follow_up_topical_treatment

This photos demonstrate the recovery that took place in one month. The fish is well on its way to recover. The lesion is almost fully covered by a gel-like re growth of the skin. Note the one dead scale that was not removed during the original treatment. It is now covered with algae and obviously an obstacle in the healing process. It had to be removed this time and another topical treatment was applied.

The next two photos are also pressing home the point that a dead scale can cause problems if left to recocover on its own accord

infected_dead_scale_small infected_dead_scale_removed_small

Home remedies

In line with Murphy’s Law you will notice an ulcer and discover that there is no topical treatment medication available in the medicine box. Sometimes newcomers to the hobby that have not encountered the problem of ulcers before will not be sure how to approach the problem.


The following medication may be applied as a first line of defence, but the correct proprietary medication will always give better results.

Gentian Violet
Hydrogen peroxide, 3% volume diluted with 50% water
Malachite Green solution
Acriflavine solution
Wound powder
Savlon diluted
Potassium permanganate solution
Clove oil (for anaesthetic)
Methylene Blue solution

The wound is prepared exactly as described above. After drying the wound with a paper towel one of the above medication that may be available is applied to the wound. Wait about 30 seconds for the medication to sterilise the wound and dab it dry again. The wound can now be sealed with Orabase, Friar’s Balsam or propolis and dried before releasing the fish.

Snake oil

Even seasoned koi keepers sometimes call me after applying some snake oil that another hobbyist has recommended, causing severe damage to the fish. Please note that the tissue in a koi’s fins and epidermal layer are very delicate and should not be compared to mammals. Undiluted Hydrogen Peroxide, Potassium Permanganate paste, Salt paste and formalin are really harsh on the fish’s tissues and should be avoided. These chemicals will definitely sterilise the wound/ulcer but it will cause severe chemical burns that will take longer to heal than the original wound itself. The effect of these chemicals on the skin, open sores or fins of a koi may be not as severe but it is very similar to pouring salt on a snail!

No response to treatments

There may be occasions when an ulcer will not respond to treatment, even if the topical treatment is supplemented by an antibiotic injection. The reason for such occurrence can vary but the most common reasons are:

Low temperatures
The medication may have reached the expiry date or has lost its potency
The bacteria may be of a strain resistant to the topical treatment/antibiotics.
There may be cases where topical treatment will yield positive results and the ulcer will disappear, only to reappear at a later stage.

Most serious koi keepers have encountered one or all the results mentioned above. The best advice I can give in this regard is to gradually heat the quarantine pond up to at least 18, but preferably 24 degrees Celsius. If there is still no positive response, the bacteria may either be resistant to the treatment that you apply or it should be approached from the inside as well as from the outside by injections and topical treatments.

If you have heated the pond up and treated the fish with injections as well as with topical treatments, then it will be time to panic. At this stage it will be necessary to take some swabs and with the assistance of a koi health expert or your local Veterinarian send it to a laboratory for a bacterial culture growth and sensitivity test. The results will indicate what specific antibiotics should be used. Let me assure you if you do not give these prescribed injections diligently, the bacteria may become resistant again and the process will start from scratch.

Last Updated on Wednesday, 29 February 2012 12:55