Koi Ulcers

While an Unattractive Affliction, Koi Ulcers are Totally Treatable and Preventable

As a responsible pond owner, the health and wellbeing of your Koi should be priority number one. There’s a long list of things that can be harmful to your fish, but one of the most common and potentially consequential is Koi ulcers.

A group of two types of Koi in a swimming pool
The Greatest Preventative Medication for Koi Ulcers is Adequate Water Quality and a Healthy Diet.

This nasty skin affliction can affect Koi fish of all different varieties, environments, and stages of life. While ulcers may start as relatively minor, they can get quite nasty-even life-threatening-if left untreated.

In this article, we’ll provide some general information on Koi ulcers and how to spot them, along with tips for treating ulcerative fish without help from a vet.

What Are Ulcers?

Ulcers are boil-like sores that form on the skin of Koi and other species of fish. They’re recognizable by their bright red color and inflamed appearance, though they can also be pink, orange, brown, or even bluish in hue.

A Group of Koi Fish - One being Chagoi
Unlike These Healthy Koi, Some Koi Have Visible Ulcers That are Reddened Edges in Opened Skin Areas

Ulcers vary in size, shape, and location, but they show up most often on the dorsal region, around the fins, or even on or inside the gills. Despite their alarming and unpleasant features, ulcers aren’t always apparent. This is especially true when they develop on a fish’s underbelly. This covert factor makes them even more troubling.

If you’ve ever had a stomach ulcer, you know just how terrible these sorts of sores can be. The same is true for Koi, whose quality of life often declines following the formation of ulcers.

What Causes Koi Ulcers?

Koi ulcer disease (KUD) is almost always the result of bacterial infection-more specifically, a skin infection by the pathogenic bacterium Aeromonas salmonicida, which typically targets salmonids.

This bacterium, and many others like it, thrives in dirty, nutrient-rich water like the kind found in Koi ponds.

Two different Butterfly Koi Fish
To Keep Your Koi Healthy like These Koi, be sure to Provide it with a Healthy Diet

If a fish with a wound or a weakened immune system comes into contact with A. salmonicida, the bacterium can penetrate its mucoid slime layer and scales and make its way into the skin beneath. The bacteria then feed on the exposed skin cells, eventually producing raw, angry-looking lesions.

Though A. salmonicida is the usual suspect in cases of KUD, ichthyologists and veterinary scientists have identified other conditions that might open the door for bacteria.


Parasites like flukes, anchor worms, costia, and ichthyophthirius multifiliis (or “ich” for short) can wreak havoc on a Koi’s immune system. With its natural defenses lowered, the fish can become easy prey for pathogenic bacteria like Aeromonas salmonicida.

A photo of Doistu Koi with Black background
These Koi are Healthy as the Water Used is Filtered to Avoid Respiratory Stress and Parasites


Female fish tend to rub and scrape against rough pond surfaces during spawning as they release their eggs. The irritation that this sort of activity creates can leave them vulnerable to infection. To make matters worse, spawning also tends to have a negative impact on water quality.

Poor Water Quality

High concentrations of waste products like ammonia and nitrites can place a lot of stress on the Koi’s immune system. If you’re not monitoring the water quality in your pond, you could be allowing your Koi to fall victim to ulcers and other ailments.

Complications of Ulcers

Koi ulcers aren’t just unsightly and uncomfortable-they also come with a host of other issues. For example, fish suffering from severe KUD might also be at risk of:

  • Dropsy. Dropsy is a type of swelling visible in the abdomen of fish with bacterial infections. It may be caused by the accumulation of fluid or by the enlargement of the internal organs themselves. Dropsy is a severe condition and can even be fatal.
  • Fungal infections. Fungal infections occur when opportunistic fungi establish themselves on the damaged tissue surrounding ulcers. Needless to say, developing both bacterial and fungal infections makes things twice as bad for the fish.
  • Internal infections. If a bacterial or fungal infection gets advanced enough, it can spread to the fish’s blood and internal organs, putting its life in jeopardy. At this point, veterinary intervention is usually necessary.
  • Death. Sadly, death is often the outcome when Koi ulcers go unaddressed.
Two Types of Chinese Koi Fish - One with Black Markings and One with Orange Markings
Unlike these Koi, Some Injuries Can and Will Most Likely Get Infected Easily When Their Natural Defenses are Compromised

How to Treat Koi Ulcers

Fortunately, you’re not powerless in the face of KUD.

At-home Koi ulcer treatment is surprisingly simple. You don’t need a degree in aquatic veterinary medicine, nor do you need expensive tools or extensive training. All you need are a few basic materials and an idea of what might be causing the issue.

With that said, it’s always best to have a qualified aquatic veterinarian take a look at your Koi anytime you think there might be something wrong with them. However, if you lack the time or funds for a house call or find yourself dealing with ulcers frequently, it can be helpful to know how to remedy them yourself.

Step 1 - Eliminate Environmental Stressors

Whenever you suspect that water quality may be to blame for some illness or ailment, you should always make it a point to correct the problem before you do anything else.

Depending on the particulars of your pond, this could mean cleaning your pond, changing your Koi’s feeding regimen, or finding a more efficient filtration solution. 

A variety of Different Koi Fish in a pond
Always Check Your Koi for Any Illnesses to Keep them as Healthy as These Koi Fish

Providing much-needed oxygen via aeration in the form of fountains, streams, waterfalls, or agitation devices can also do wonders for improving water quality. Mild cases of KUD sometimes clear up on their own once the underlying cause has been dealt with.

You’ll know your Koi’s ulcers are getting better when they go from a fiery red color to soft pink with white edges, which is an indication that the wound is beginning to heal. Severe ulcers may cause permanent scarring, and the fish may or may not be able to regrow the scales in the affected area.

Step 2 - Treat the Wounds Manually

To clean a Koi ulcer and protect the site from further irritation, you’ll need a large bucket, a net, a cotton swab, some hydrogen peroxide, topical betadine, and a little clove oil or some other means of sedating the affected fish. Here’s how to perform ulcer swab cleans for your Koi: 

  1. Fill your bucket with fresh water. This water will serve as a temporary environment for your Koi while you administer aid.
  2. Carefully scoop up the afflicted Koi with your net. Place the net over the opening of the bucket so that the fish remains in partial contact with the water at all times.
  3. If possible, anesthetize your Koi to limit the fright and discomfort it feels throughout the treatment process. You can do this by combining nine parts ethanol with 1 part all-natural clove oil, then adding the mixture to the water in your bucket.
  4. Dip a cotton swab in hydrogen peroxide and use it to debride the ulcer. “Debride” simply means to remove dead or damaged tissue to uncover the health tissue underneath.
  5. Once you’ve cleaned up the wound, rinse it liberally with betadine. This common antiseptic is readily available at most drugstores, as well as some pet supply shops.
  6. Coat the ulcer with a thin, even layer of medicated gel powder. Products like Bio Bandage and Fin & Body Cure are well-suited for this task. When applied on top of the betadine, the powder will thicken into a gel and help seal off the wound.
  7. Squirt the ulcer with betadine one more time to make sure any lingering bacteria won’t cause reinfection.
  8. Release the Koi fish back into its pond and keep a close eye on it over the next several days. It may take 2-5 days for an ulcer to begin showing signs of healing.
  9. If the condition hasn’t improved after about a week, I strongly recommend scheduling an appointment with a veterinary specialist.
A Koi Swimming Pool in the Backyard filled with Different Varieties of Koi
To Maintain Good Koi Health, Make Sure You Have Good Water Quality

Note: If you opt to use Fin & Body Cure, keep in mind that it’s medicated and may therefore change the color and chemical composition of your pond water slightly.

Frank Salvatore

In 2015 our family moved into a house south of Denver, Colorado with a  koi pond.  Since that time I've learned to really enjoy the koi fish and the pond. This blog is dedicated to providing helpful hints and information for koi pond hobbyists - as well as those of you who just inherited a koi pond and are thinking NOW WHAT?

About Me

Hey there - I'm Frank Salvatore. In 2015 our family moved into a house south of Denver, Colorado that had a Koi pond. The problem was I knew absolutely NOTHING about koi ponds.

This blog is dedicated to providing helpful hints and information for koi pond hobbyists - as well as those of you who just inherited a koi pond and are thinking NOW WHAT?
Learn More About Me

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