Koi or gold fish has ulcers

Unfortunately, the numbers of koi suffering from ulcers has increased over recent years. The majority of koi keepers will have experienced this problem and had mixed results in successfully treating koi with ulcers. In many respects, an ulcer represents a real challenge to a koi keeper as they are notoriously difficult to treat, especially if they are not noticed sufficiently early. Recognising that ulcers are on the increase and they are difficult to treat, it is prudent to be as well informed about their control as possible.

What are pond fish or koi ulcers?

Ulcers are ‘holes’ that develop on the exterior of a fish (usually on the flanks or underneath) where an internal bacterial infection at first causes a localised area of scales to redden and become raised. The rampant and infectious nature of the infectious bacteria cause the scales soon to drop off, leaving an area of delicate, sensitive and unprotected tissue exposed to the water, easily visible to the naked eye.

Ulcers are like most other koi diseases in that there will be an identifiable cause, with a koi’s overall susceptibility to disease being increased through stress. Besides the general stressors such as unstable or long-term poor water quality and rough handling, specific incidents may also cause koi to develop an ulcer. Although ulcers generally develop from an internal infection, they can also be caused by external knocks or abrasions becoming infected. This is especially true of koi starting to become active in spring having spent months over the winter resting on the abrasive pond bottom. This can often lead to a ‘bed sore’ type ulcer affecting the ‘belly’ of a koi.


There are several courses of action that can be effective on an ulcerated koi to help reduce stress while treating the site of the ulcer itself.

Reducing stress

An open ulcer allows water to enter freely into the koi’s tissues. Water continually moves from an area of high water concentration (pond water) to low water concentration (fish tissue). This puts an extra burden on the fish’s physiology as it has to respond by increased kidney activity, excreting the excess water as quickly as possible.

The addition of salt to the pond water reduces the imbalance of water concentrations between the pond and the fish, reducing the rate at which water enters the fish. This reduces the work load experienced by the kidneys and reduces the overall stress on the fish. Pond salt should be added at a rate of 0.1% i.e. 1kg salt per 1000 litres of water or approx. 1.5 oz per 10 gallons.

Unfortunately, the addition of salt may retard the growth of some aquatic plants and care must be taken to remember that once salt has been added to a pond the only way of removing it is through partial water changes.

Treating the ulcer.

The addition of salt to the pond water also acts as an antibacterial which is handy, but extra supportive treatment is required to enable a koi to completely heal. A koi keeper should aim at treating the infection and relying on the koi to heal itself.

Topical treatment

The regular application of an effective antibacterial treatment directly onto the open wound can be very effective. Solutions containing malachite green or other broad-spectrum antibacterials are readily available. Treatment is more effective if the ulcer is dabbed dry in the early days of treatment to allow the medication to be absorbed deeply into the affected tissue. Later on in the treatment, and to enable healing, the dropping of the solution directly onto the wound is advisable.

In addition, a gel or starch-based poultice mix may be applied onto the open wound in the early days of treatment to prolong the contact between the antibacterial action and the infection. Care must also be taken when netting and handling an ulcerated koi between treatment so as not to aggravate the wound or the fish. The use of an anaesthetic may be required for repeated close work on an ulcer.


It is becoming increasingly difficult for Vets. to find antibiotics that are still effective against an increasing range of antibiotic resistant bacteria.

However, if it is possible to find a Vet. who is willing to prescribe and administer an effective antibiotic, then the chances of a full recovery are greatly increased. This is likely to be a costly method, involving a series of antibiotic injections.

The above methods of treating a koi with an ulcer allows a 3 pronged attack which will be effective in allowing the fish to heal itself if used in conjunction with ideal water conditions, excellent nutrition and a restful environment.

However, if scales are lost through ulceration, although the koi’s skin will completely regenerate to show the original colour, the scales will not grow back, leaving a permanent mark at the position of the ulcer.

Kill blanketweed and string algae.