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905-715-2447 Summer Hours: Open Wednesday to Saturday 10:00 am - 5:oo pm 3301 Sideroad 10, Bradford, ON Canada

How To Treat Ulcers In Koi & Goldfish

How To Treat Ulcers In Koi And Goldfish

Ulcers (red sores or spots) are fairly common in koi goldfish ponds and can be caused by a several different things. You need to treat ulcers in koi and pond fish as soon as possible.

Ulcers are caused by bacteria (Aeromonas sp.)that have penetrated through the koi’s defences (mucous (slime coat), skin and scales). Ulcers often start out very small, but if left untreated can become large enough to kill the fish.

If you need to treat ulcers koi or other pond fish it is important determine what likely caused it in the first place. In most cases ulcers are caused by parasitic infection,  poor water quality and even spawning.


  • Parasitic Infection – parasites such as anchor worm, flukes, costia even ick are common precursors to bacterial ulcers. These parasites break through the mucous and skin of the fish leaving an entry path for bacteria.
  • Water Quality – is another very common cause of ulcers. When fish are exposed to poor water quality (high ammonia or nitrites) they become stressed and their immune systems are weakened. As a result, parasites and bacteria have a better chance of breaking through the fish’s defences.
  • Spawning – is also a reoccurring cause of ulcers. If you have ever witnessed your fish spawning you soon realize that it can get quite rough, especially for the females. Fish often lose scales and get scrapes from rubbing against pots, rocks or pumps in the pond. Spawning can last 3 or 4 days which can take its toll on the fish as well as the water quality, another great chance for bacteria to invade.


If you determine you have an parasite problem in the pond, it is important to treat for parasites immediately, as well as the treat ulcers in koi.

how to treat ulcers in koi and goldfish
The fish above show signs of an ulcer. Note the red infected tissue around the perimeter of the affect area. The ucler on this koi was caught early enough so that it made a full recovery with treatment .

If left undetected for long, often develop a secondary infection of fungus growing from the dead tissue of the wound. Beneath the green/gray fuzz of the fungus is an advanced ulcer

There are many different treatments for ulcers, some work better than others, but you will always have the best results if you catch it early and treat immediately.


  • Injections – Baytril, Nuflor
  • Antibiotic Pond treatments – Melafix, Pimafix, Oxolium powder, Bifuran
  • Topical Treatment – iodine, potassium permanganate or hydrogen peroxide
  • Medicated Foods – Due to the regulations of medicated fish foods, you will have to make your own medicated fish food .
  • Dips or Baths – Tricide Neo



Although injections are probably the most effective way to treat bacterial infections in koi and goldfish, they are not very practical for the average hobbyist. If you are interested in injections, you will need to see your local veterinarian.

Topical Treatments

These are often very effective treatments for ulcers. Typically, one uses iodine, potassium permanganate or hydrogen peroxide to kill the bacteria, clean and cauterize the wound.

Antibiotic Pond Treatments

Treating a pond with antibiotic sounds like a great way to combat bacterial infections, but it does have some drawbacks. They can be very expensive in larger ponds and they will not only kill the bad bacteria in the ulcer but also the good bacteria in the filter, resulting in poor water quality. This is where natural remedies such as Melafix works well.  We often use it here in our fish holding systems when receiving new koi and goldfish because does help to heal split fins and scrapes caused during transport and prevents infections from occurring. Pimafix is another effective product that is similar to Melafix that works well to treat both  fungus and bacteria.

Dips or Baths

Medicated dips or baths can work well to treat ulcers in koi, Tricide Neo is extremely effective at conquering ulcers. However the only downside is it must be used daily for 4 to 5 days and the duration of the bath must be times precisely. If you don’t have a hospital or quarantine tank it is very difficult and stressful to catch a fish from the pond repeatedly.

Medicated Foods

Using a good medicated food is a simple and somewhat effective way to get medication directly ‘into’ the fish, as long as it is eating. There are recipes available online for making your own medicated food.

Our  Suggested Method To Treat Ulcers In Koi

Equipment Required:

  • Net
  • Oil of cloves (optional)
  • Q-Tips
  • Towel
  • Hydrogen Peroxide
  • Melafix


Steps To Treat Ulcers in Koi:

  1. Catch the fish and transfer it to an appropriately sized container. At this point you may choose to anesthetize the fish to make handling it easier, especially if it is over 12” long. Oil of cloves works very well and is readily available. …..
  2. Take a Q-tip, dip it in the peroxide and begin to gently but firmly rubbing the ulcer and the area around the ulcer. Scrub in a head –to-tail direction, don’t be scared if some scales come off while working. You want to be very thorough, rub under and around the scales so that any ‘hiding’ bacteria is killed.
  3. You may want to return the fish to the container periodically if it is a large ulcer or the fish has more than one.
  4. Finally, dry the area and apply and antibiotic solution such as bio-bandage to help protect the ulcer from further infection. This step can be repeated daily as needed.
  5. Return the fish to the pond or hospital tank.
  6. Treat pond or hospital tank daily with Melafix as directed

*please note that swabbing the ulcer should only be done once, repeatedly scrubbing the ulcer will prevent healing from occurring.


  • Only treat red, bloody wounds. Look closely at the ulcer before you catch the fish. If the ulcer is pink with a white rim do not scrub it with peroxide or iodine – it is already healing on its own


You should see an improvement in the appearance of the ulcer in 2 to 3 days if the treatment is working. The rough red tissue should gradually turn  a pinkish colour, then white and smooth. It may take several weeks to months for the healing area to return to the natural colour of the fish. A noticeable scar may remain.