What are ulcers and how can I treat them?

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William Wildgoose BVMS CertFHP MRCVS is a veterinary surgeon at the Midland Veterinary Surgery in London and specialises in ornamental fish. Here he explains what causes ulcers and how you can treat them.

What are ulcers?

These are one of the most common lesions in Koi and affect most external parts, including the fins and mouth. It is important to check all fish, particularly the belly of pond fish, because this area may be badly ulcerated yet may not be obvious until too late.

What causes them?

Most ulcers are caused by bacterial infections that are commonly found in ‘healthy’ ponds but cause little harm until the fish are stressed by poor water quality or environmental conditions.

Increasing parasite burdens and substantial temperature fluctuations may also stress fish indirectly and make them more susceptible to bacterial infection and ulcers.

Are fungal diseases linked?

Fungal infections usually occur after the ulcers have developed rather than cause ulcers directly. Therefore it is vital that water quality is improved by carrying out a 30% water change every three or four days for two or three weeks.

My water tests are proving fine, so what else could cause the problem?

Even when basic water tests for ammonia, nitrite and nitrate might suggest that the water quality is good, there are many other substances in pond water that may cause problems. These include hydrogen sulphide from decomposing organic matter. If in doubt, change some water.

What about treatments?

After the first water change, adding salt at 1.5 grams per litre (or quarter of an ounce per gallon) helps wounds to heal in freshwater fish by reducing the osmotic effects of water seeping into the ulcer and by having a mild disinfectant action. The salt content should be maintained after each water change by adding 30% of the original amount of salt.

This can be checked and monitored by using a hydrometer, or, more accurately, a conductivity meter to measure dissolved solids, like salt.  Adding an antibacterial product to the water such as Virkon, or one of the many proprietary brands, may be helpful.

What if I find that the ulcers are extensive?

If this is the case, then the fish will require an anaesthetic and the wounds surgically cleaned up, disinfected and dressed with a waterproof compound.

If only one or two fish are affected then they may benefit from being kept in a heated isolation tank, but this should be set up carefully, be of an adequate size and water quality monitored frequently. Heating the water to 25°C/77°F improves wound healing, particularly if pond water temperatures are low in spring and autumn. However, if several fish are affected then they are best treated in the pond, where water conditions will be more stable.

Ulcerated fish often require antibiotics given by injection and, if more than a few of your fish are affected, all of them should be given antibiotic medicated food for two weeks. Contact your local veterinary surgeon for professional advice.

This item was first published in the November 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.