Koi bring beauty and tranquility to your pond, and keeping them healthy requires attention to common koi diseases for early detection, and proper care. This guide covers the most common viral and bacterial diseases affecting koi, their symptoms, and treatment options. Key viral diseases include Koi Herpesvirus (KHV) and Spring Viremia of Carp (SVC), both serious and often without cures. Bacterial infections such as ulcers, Columnaris, and dropsy are more common but generally treatable with prompt action.
Preventing illness involves maintaining excellent water quality, quarantining new fish, avoiding overfeeding, and feeding a high-quality, digestible food like Kenzen Koi Food, which supports immunity and reduces pond waste. Early observation of behavioral and physical changes is critical for effective intervention. Using proper hospital tank setups, salt treatments, topical care, and medicated food can aid recovery when needed.
Healthy koi thrive on balanced nutrition, clean water, and attentive care — all essential to sustaining their vibrant colors, longevity, and well-being. Kenzen Koi Food supports these goals by promoting strong immunity, cleaner water, and overall pond health.
Viral Diseases in Koi: What You Need to Know
Here at Kenzen Koi, we understand how much joy and serenity koi bring to your life—and we’re just as passionate about keeping them healthy as you are. With their vibrant colors, calming presence, and unique personalities, koi bring beauty and joy to our lives. That’s why we’re so passionate about helping koi owners protect their fish from disease and ensure they thrive year-round.
In this guide, we’ll walk you through the most common viral and bacterial diseases koi may face, explain how to recognize and treat them, and share how proper care—including nutrition—can make a world of difference.
Viral diseases may be less common than bacterial infections, but they’re often more severe—and harder to treat. Prevention and early detection are critical.
Koi Herpesvirus (KHV)
KHV is one of the most serious diseases affecting koi. It spreads quickly and can lead to high mortality rates, especially in water temperatures between 72–81°F (22–27°C).
Symptoms:
- Lethargy
- Gasping at the water surface
- Pale or frayed gills
- Skin lesions, sunken eyes
Important to Know:
There is no known cure for KHV. Infected fish often require euthanasia, and outbreaks can devastate entire collections.
Prevention Tips:
- Always quarantine new fish for at least 4 weeks
- Buy koi from reputable, KHV-free certified sources
- Keep water conditions stable and healthy
Spring Viremia of Carp (SVC)
SVC tends to emerge in the spring when water temperatures are still cool (50–65°F / 10–18°C). It’s also a reportable disease in many countries, meaning authorities must be notified of outbreaks.
Symptoms:
- Swollen belly
- Hemorrhaging (visible red spots or streaks)
- Lethargy and abnormal swimming
Treatment:
There is no direct cure, but supportive care, isolation, and temperature control can help. Focus on minimizing stress and preventing secondary bacterial infections.
Bacterial Diseases: More Common, But Often Treatable
Bacteria are naturally present in pond environments, but stress, injury, or poor water conditions can turn harmless strains into dangerous infections.
Aeromonas and Pseudomonas
These bacteria are responsible for ulcer disease, a common issue especially in spring and fall.
Symptoms:
- Red ulcers or sores
- Frayed fins
- Swollen body or signs of dropsy
Kenzen Koi’s Advice:
Improve water quality immediately, isolate affected koi, and treat ulcers with topical antiseptics. A course of antibiotics—under veterinary supervision—may be needed in more advanced cases.
Columnaris (Also Known as Cotton Wool Disease)
Often confused with fungus, Columnaris presents as cottony white patches, especially around the mouth and gills. It spreads fast in warmer water above 65°F (18°C).
Symptoms:
- Grayish-white lesions
- Difficulty breathing
- Fin and tail rot
Treatment:
Salt dips, antibacterial pond treatments, and quarantining affected koi are usually effective. This is where prevention makes all the difference.
Dropsy
Dropsy isn’t a disease itself but a symptom—usually of internal bacterial infection or organ failure. It’s serious and often fatal if not treated early.
Symptoms:
- Bloated body with protruding scales (pinecone appearance)
- Lethargy, loss of appetite
- Bulging eyes
What You Can Do:
- Move the fish to a hospital tank
- Treat with Epsom salt and antibacterial medications
- Provide supportive care, including high-quality nutrition
Early Detection Saves Lives
The earlier you catch illness, the better your chances of helping your koi recover. Watch closely for these early warning signs:
- Flashing (rubbing on objects)
- Lethargy or hiding
- Changes in appetite
- Physical changes (spots, ulcers, swollen body)
Regular observation—especially during seasonal temperature shifts—is a key part of koi health management.
Common Causes of Illness: Mistakes to Avoid
Here are some of the most common (and avoidable) triggers of disease:
- Overfeeding or Poor-Quality Food
Low-quality koi food often contains fillers that aren’t easily digested. This leads to poor nutrient absorption, more waste in the water, and added strain on your pond’s filtration system.
Solution: Feed a premium, highly digestible food like Kenzen Koi Food, which supports strong immunity while reducing water pollution.
- Neglecting Water Quality
Dirty water is a breeding ground for pathogens. Even short-term ammonia spikes or low oxygen levels can stress fish and leave them vulnerable to disease.
Solution: Test your water regularly and keep your filters clean. Aim for:
- Ammonia: 0 ppm
- Nitrite: 0 ppm
- Nitrate: <40 ppm
- pH: 7.0–8.2
- Skipping Quarantine
Introducing new koi without quarantine is one of the fastest ways disease spreads.
Solution: Quarantine new arrivals for at least 4–6 weeks and monitor closely for any signs of illness.
Treatment Options: Tools and Techniques
Hospital Tank Setup
- Use a 100–300 gallon quarantine tub with filtration and aeration
- Match temperature to your pond
- Change 25–50% of the water daily during treatment
Salt Treatments
Salt can help with parasites and minor bacterial infections:
- Use pure pond salt (non-iodized)
- Dosage: 0.3% (3 lbs per 100 gallons), added over 48 hours
- Monitor your fish for signs of stress
Topical Treatments
For ulcers or wounds:
- Dry the area and apply iodine or a commercial koi ulcer treatment
- Avoid handling koi roughly—use a wet towel and calm environment
Medicated Food and Antibiotics
Feeding medicated food can help with internal infections, but it’s only effective if the koi are still eating. Injectable antibiotics may be necessary for advanced cases but require vet assistance.
Prevention Is Better Than Cure—and It Starts with Nutrition
Your koi’s first line of defense isn’t a medication—it’s their immune system. And that starts with what they eat every day.
At Kenzen Koi, we’ve spent years perfecting a formula that supports every aspect of koi health, from digestion to immunity and even water cleanliness.
Why Kenzen Koi Food Stands Out:
- Highly Digestible Proteins: Less waste means cleaner water and lower ammonia levels
- Fats that remain liquid at any water temperature.
- Digestible fats: Many Fats found in grain are solid at low water temps and become undigestible.
- Immune Boosters: Includes our exclusive blend of Bioceuticals
- Balanced Nutrition: With optimal levels of proteins, fats, carbs, and essential vitamins
- No Fillers or Artificial Colors: Just clean, science-backed ingredients that koi actually need
The Kenzen Difference: Cleaner Water, Healthier Fish
Feeding Kenzen doesn’t just help your koi—it helps your pond, too. Many koi owners are surprised at how much clearer their water becomes after switching to Kenzen. That’s because our food is designed to reduce waste and maintain water quality, which in turn:
- Lowers the risk of infection
- Reduces algae growth
- Makes your filtration system more effective
Think of it as part of your pond’s overall health plan. Good food equals less cleanup, fewer problems, and more time enjoying your fish.
We’re Here for You and Your Koi
At Kenzen Koi, our mission is simple: to help koi live longer, healthier, and more vibrant lives. Disease prevention isn’t about luck—it’s about giving your koi the best possible environment, the right nutrition, and attentive daily care.
To recap:
- Monitor your koi daily
- Test and maintain water quality
- Quarantine new fish
- Act quickly at the first sign of illness
- Feed a scientifically formulated, immune-supportive food like Kenzen Koi
Whether you’re just starting your koi journey or you’ve been in the hobby for decades, we’re honored to be part of your pond’s success. If you have questions, we’re always here to help—with knowledge, experience, and food you can trust.
Healthy koi start with Kenzen.