Understanding Ulcers on Koi: Prevention & Treatment

By Tri-Star Koi

One of the more common and concerning health issues koi keepers may face is ulceration—open sores or lesions that appear on a koi’s body, often red, raw, and sometimes raised. These ulcers are more than just surface wounds—they're usually signs of a deeper issue with water quality, parasites, or the koi’s immune system. Before addressing this issue, it's important to understand that without testing the specific bacteria involved, it would be irresponsible to recommend any injections or medications. This article is intended to provide general care guidelines and best practices, rather than offer treatment advice in the absence of proper bacterial identification.

At Tri-Star Koi, we take ulcer prevention seriously because once an ulcer appears, it requires immediate attention. The good news is that with early detection, proper care, and good pond practices, you can prevent most ulcer cases—and treat them effectively when they do happen.

What Causes Ulcers in Koi?

Ulcers are usually the result of bacterial infections that enter through damaged skin. But the real cause often starts earlier with one or more of the following:

• External parasites (like flukes or costia) that break the skin barrier

• Poor water quality, especially high ammonia, nitrites, or pH swings

• Stress from overcrowding, rapid temperature changes, or transport

• Physical injuries from handling, spawning, or sharp objects in the pond

• Weakened immune systems, often due to stress or poor nutrition

Once the koi's skin is compromised, opportunistic bacteria—often Aeromonas or Pseudomonas—take hold, and ulcers begin to form.

How to Prevent Ulcers

Prevention always starts with solid pond management and proactive care. Here are a few key practices we follow at Tri-Star Koi:

• Maintain excellent water quality: Regularly test for ammonia, nitrite, nitrate, pH, and KH. Stable water is the koi’s first defense against disease. At our farm, we perform daily water changes as part of our regular koi care routine. While we understand this may not be feasible for all hobbyists, it's the approach that works best for us and helps maintain optimal water quality for our fish.

• Quarantine new arrivals: Never introduce new koi directly into your pond. Quarantine helps you monitor and treat fish before they meet your main stock.

• Treat parasites early: Many ulcers begin because of parasite damage. A proper parasite prevention program can go a long way in keeping ulcers away. At the farm, we routinely perform weekly skin scrapes on our koi to monitor for parasites. While infestations are rare, when they do occur, we treat them promptly and appropriately. Parasites are something every koi keeper will encounter at some point—what matters most is being prepared and maintaining high water quality to minimize risk and support recovery.

• Feed a high-quality diet: Nutrition supports immune health. Avoid overfeeding and choose koi food appropriate for the season.

• Avoid overcrowding: Too many fish in one space increases stress and disease risk.

• Remove sharp or dangerous objects from the pond that koi can injure themselves on.

Signs Your Koi Might Have an Ulcer

Early signs include:

• A red or raw patch on the body or fins

• Raised sores or open wounds

• Lethargy or isolating from other fish

• Loss of appetite

• Flicking, rubbing, or flashing behavior (if parasites are involved)

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How to Treat Koi Ulcers

If you notice an ulcer forming, quick action can prevent it from becoming life-threatening. Here’s a general approach:

1. Move the Fish to Quarantine

Remove the affected koi from the main pond and place it in a clean, well-aerated quarantine tank. This prevents infection from spreading and allows for closer monitoring.

2. Improve Water Quality

Test the water in both your pond and the quarantine tank. If there are any water quality issues, address them immediately.

3. Topical Treatment

Gently sedate the koi (only if you’re experienced or working with a professional), then clean the ulcer area with hydrogen peroxide or iodine. Carefully remove any infected scales and use a Q-tip to apply hydrogen peroxide or iodine directly to the affected area. This helps localize and control the infection.

4. Salt

Add salt to the pond or water (0.5%- 0.6%) to reduce stress and help fight bacteria. This should be done under controlled conditions.

5.) Water changes

In most cases, simply cleaning the wound and maintaining pristine water conditions allows your koi to heal on its own. We recommend performing two 10–20% water changes per week until you see signs of recovery.

6.) Treat for any Parasites

Most importantly, if your koi developed an ulcer due to a parasite, it's crucial to identify and eliminate the parasite. Ulcerations often begin as parasitic infections, which can then lead to secondary bacterial infections. By maintaining pristine water quality, keeping fish load manageable, and performing regular weekly water changes, you significantly reduce the risk of parasite outbreaks.

When to Seek Help

If ulcers spread, don’t heal within a few days, or the koi starts showing signs of systemic illness (clamped fins, heavy breathing, laying on the bottom), it's time to seek professional help. Catching ulcers early makes a big difference in recovery success.

Ulcers can look scary, but they’re manageable with the right response and care. The real key is prevention—a healthy environment, stress-free koi, and regular parasite management will keep most ulcers at bay.

At Tri-Star Koi, we believe in proactive care over reactive treatment. If you ever need advice on ulcer treatment, quarantine procedures, or koi health in general, don’t hesitate to reach out.

— Marlon @ Tri-Star Koi