Trouble in the Water?
Ever notice your koi fish acting strange or looking a little off? Wondering what those white spots or frayed fins really mean?
In this post, you’ll learn how to recognize the most common koi diseases, what causes them, and how to treat or prevent outbreaks in your pond.
By the end, you’ll feel more confident in protecting your koi and keeping your pond a peaceful, healthy space to enjoy.
Disclaimer
The information in this post is for general educational purposes only and should not be taken as veterinary or medical advice. Morganton Ponds & Hardscapes is not a licensed veterinarian, and readers should always consult a qualified aquatic specialist or vet before diagnosing or treating koi health issues.
Common Koi Fish Diseases (With Symptoms)
| Disease Name | Main Symptoms | Common Causes | Typical Treatments |
|---|---|---|---|
| Ich (White Spot) | White spots, flashing, clamped fins | Protozoan parasite, stress, or poor water quality | Raise water temp, anti-parasitic treatment, UV sterilizer |
| Fin Rot | Frayed/discolored fins, redness | Bacterial infection, poor water | Antibacterial treatments, water changes |
| Ulcers | Open red sores, cloudy eyes, lethargy | Bacteria entering damaged skin | Antibiotics, salt baths, and clean the pond |
| Dropsy | Swollen belly, pinecone-like scales | Internal bacterial infection | Antibiotics, improved water quality |
| Anchor Worm | Visible worms, red sores, and scratching | Crustacean parasite | Manual removal, anti-parasitic treatment |
| Fish Lice | Itching, flashing, small parasites on the skin | Parasite introduced via new fish | Anti-parasitic medication, pond cleanup |
| Gill Flukes | Labored breathing, excess mucus, flashing | Flatworm parasite, poor oxygen | Praziquantel, improved aeration |
| Swim Bladder Disease | Buoyancy issues, floating or sinking | Poor diet, infection, injury | Fasting, shelled peas, water temp adjustment |
| Koi Herpesvirus | Erratic swimming, sunken eyes, discolored gills | Highly contagious virus | No cure, isolate infected fish |
| Carp Pox | Waxy white growths on the skin | Viral; worsens in cold water | No cure, isolate fish, improve conditions |
| Trichodina | Flashing, spiderweb lesions, gill damage | Protozoan parasite | Salt treatment. Improve water quality |
| Columnaris | Gray film, frayed fins, sores on mouth/body | Bacterial infection | Antibiotics, water quality management |
| Epistylis | White tufts on wounds, similar to fungus | Protozoan parasite, dirty water | Salt baths, clean pond |
| Lymphocystis | Cauliflower-like skin growths | Viral infection | No cure, isolate fish |
| Costia | White patches, lethargy, fast breathing | Protozoan parasite | Salt or anti-parasitic treatments |
| Chilodonella | Clamped fins, grey patches, gasping | Protozoan parasite | Anti-parasitic treatment, pond aeration |
| Aeromonas | Ulcers, red streaks, lethargy | Bacterial infection in poor conditions | Antibiotics, pond cleanout, water testing |
Ich (White Spot Disease)
Ich is one of the most well-known koi fish diseases and shows up as tiny white spots on the body and fins, much like the fine, grainy residue you might see along the sun-warmed stones near the Catawba River Greenway.
It’s caused by a protozoan parasite that attacks when koi are stressed or living in poor water quality.
Koi will often rub or flash against rocks and swim erratically in discomfort.
You’ll also notice clamped fins and heavy breathing as the parasite invades the gills.
Ich spreads fast and can wipe out many fish if not addressed quickly.
Fin Rot
Fin rot is a bacterial infection that usually starts on the tips of fins and moves inward if untreated.
The fins may look frayed, discolored, or ragged.
It’s common in koi that have been injured or exposed to dirty pond water.
This fish disease can progress quickly, especially in warmer water temperatures.
It may open the door to secondary bacterial infections if not treated with medicated food or pond-safe antibacterial treatments.
Ulcers
Ulcers are open sores that can appear on the body of a sick fish.
They’re usually round with red edges and can look like raw, inflamed craters.
They often start after parasites or injuries break the skin, allowing bacteria to enter.
In some cases, these open sores are paired with cloudy eyes or clamped fins.
Koi with ulcers may isolate from other fish or rest near the pond bottom.
Treatment usually includes water changes, antibiotics, and isolation in a quarantine tank.
Dropsy
Dropsy isn’t a disease itself but a symptom of something deeper, often a bacterial infection or organ failure.
Affected koi look extremely bloated, with scales sticking out like a pinecone—hence the nickname pinecone disease.
The fish may float oddly or have difficulty swimming.
Dropsy can spread quickly throughout the pond if caused by poor water quality or overcrowding.
Immediate isolation and antibacterial treatment are needed, though the survival rate is low in late stages.
Anchor Worm and Fish Lice
Anchor worms are a type of crustacean parasite that physically embeds into the koi’s body, clinging to their skin like invasive vines wrapping around the trees in Freedom Park’s arboretum groves.
Fish lice, which are similar, attach to the skin and gills.
These small parasites can often be seen with the naked eye, and koi will often try to scrape them off by rubbing against surfaces.
Look for red sores, flashing, and visible worm-like structures.
Removing these parasites typically involves a salt bath and a pond treatment that targets adult parasites and their eggs.
Gill Flukes and Skin Flukes
These flatworm parasites attach to the gills or skin and feed on tissue, leading to gill damage, labored breathing, and mucus buildup.
Koi with gill flukes may show signs of respiratory distress, such as gasping at the surface or breathing rapidly.
Skin flukes may cause red streaks and clamped fins.
These protozoan parasites are often introduced by new fish and are difficult to spot without a microscope.
Treatment usually includes praziquantel and improved water conditions.
Swim Bladder Disease
Swim bladder disease affects the fish’s ability to control its buoyancy.
A sick koi may float sideways, sink to the pond bottom, or bob uncontrollably at the surface.
It’s often caused by poor diet, injury, or bacterial infection.
Feeding koi shelled peas or adjusting the feeding schedule can help in the early stages.
In more severe cases, isolation and supportive care are necessary to prevent long-term damage.
Koi Herpesvirus (KHV)
Koi herpesvirus is a serious disease that spreads rapidly and can kill an entire population of koi in just a few days.
It’s most active in water temperatures between 70 and 80 degrees Fahrenheit.
Symptoms include discolored gills, sunken eyes, and erratic swimming.
There is no cure once koi are infected, so prevention is critical.
Any new koi should be kept in a quarantine tank and observed closely before entering the main pond.
Carp Pox
Carp pox is a viral koi fish disease that causes waxy, white growths on the skin, similar in texture to candle wax.
Though it’s not fatal, it can spread easily and doesn’t have a cure.
Carp pox tends to show up more often in cooler water temperatures and usually fades in warm weather.
Infected fish should be isolated if lesions appear in large numbers.
Trichodina
This saucer-shaped protozoan parasite shows up under a microscope and causes lesions that look like spiderwebs on the skin.
Affected fish may flash and rub on surfaces, often scratching themselves raw.
Smaller koi are more vulnerable and may suffer gill damage.
Salt treatments are typically effective, and maintaining optimal water quality helps prevent reinfection.
Columnaris
Columnaris is a bacterial infection that can spread through the water column and attack the fins, mouth, and gill cover.
It causes grayish film on the body and ragged fin edges.
It may look like a fungal infection at first, but it’s actually a fast-spreading bacterium that thrives in stressed or overcrowded koi ponds.
Antibiotics are typically needed, along with improved aeration and water filtration.
Epistylis
Epistylis is often mistaken for fungus because of its fuzzy, white tufts, but it’s a protozoan parasite that enters open wounds or ulcers.
It usually shows up when the pond water is dirty or under-filtered.
Treatment starts with cleaning the pond and using salt baths to kill off the parasite.
Lymphocystis
This viral koi fish disease causes irregular, cauliflower-like growths on the body or fins.
It’s contagious and hard to treat, so infected fish should be separated from the rest of the pond.
While not fatal, it weakens the koi and can lead to other health problems over time.
Costia, Chilodonella, and Aeromonas
These three pathogens are common in many fish diseases.
Costia and Chilodonella are protozoan parasites that affect gill function and cause clamped fins, grey patches, or flashing.
Aeromonas is a bacterial infection that leads to ulcers, red streaks, and lethargy.
All three thrive in dirty pond water and are more likely to appear during a sudden drop in water quality or after introducing new fish.
How to Tell If Your Koi Is Sick
Behavioral Signs
Koi are active, curious fish, so when they stop acting like themselves, it’s a red flag, just as the sudden silence of birds along the Riverbend Park trail signals something is wrong.
If a fish is hovering near the pond bottom or isolating from the group, it could be feeling unwell.
A sick koi may stop eating or swim in unusual patterns like darting, floating sideways, or even spiraling.
Flashing, where koi rapidly rub against rocks or surfaces, often signals irritation from parasites or poor water quality.
Gasping at the surface is another sign that gill function is compromised, often due to gill flukes, bacterial infections, or low oxygen levels in the pond water.
Physical Symptoms
Changes to a koi’s appearance can offer clear clues about illness.
Look for clamped fins, where the fins are held tightly against the body, or frayed, ragged fin edges, which may point to fin rot.
Cloudy eyes, pale gills, or raised scales are all symptoms tied to bacterial or fungal infections.
Open sores, ulcers, or white tufts that resemble cotton may suggest a fungal infection or epistylis.
Even swollen abdomens or sunken eyes can be indicators of serious disease, such as dropsy or koi herpes virus.
These visual signs, especially when combined with unusual behavior, often mean it’s time to take action.
What Causes Koi to Get Sick?
Poor Water Quality
Poor water quality is the most common cause of illness in koi.
High levels of ammonia or nitrites, sudden pH swings, and debris buildup all create stress for pond fish and weaken their immune systems.
Even temporary dips in water quality can lead to a spike in fish diseases.
Using pond test kits regularly and keeping an eye on the filtration system can make a big difference in early detection and prevention.
When pond water turns cloudy or smells off, it’s often a sign that something’s out of balance.
Environmental Stress
Water temperatures that rise or drop too quickly can shock your koi, especially during seasonal changes.
A pond that lacks shade in the summer or freezes too deeply in winter adds to the stress.
Overcrowding is another issue. Too many fish in a small space leads to low oxygen levels, poor water circulation, and aggressive behavior.
Installing a separate aeration pump or upgrading your filtration system can ease pressure on your pond’s ecosystem and support fresh oxygen levels year-round.
Pathogen Exposure
Pathogens like parasites, bacteria, and viruses often sneak in through new fish.
Buying from a reputable koi dealer and quarantining all new koi for 2 to 4 weeks is the best line of defense.
Contaminated nets, tools, or even water plants can also carry disease.
Wildlife such as birds or frogs can spread parasites like anchor worms and fish lice between ponds.
A clean pond with proper barriers helps reduce the risk of an outbreak from outside sources.
Diagnosing Koi Fish Diseases
Observation
Start by watching your fish closely for a few minutes at different times of day.
Look for patterns: Is one fish behaving oddly, or are multiple koi affected? Are the symptoms visible across different species, or is it just the koi?
Take note of appetite, swimming style, and any signs of injury.
Often, the first clues appear during feeding time when sick fish refuse food or struggle to stay upright.
Microscopic Skin & Gill Scrapes
If you suspect parasites like gill flukes or protozoan parasites such as Trichodina, a skin or gill scrape viewed under a microscope can confirm it.
Use a clean glass slide to gently scrape mucus from the fish’s gill cover or body.
This is a precise way to catch what the naked eye can’t, but it’s only effective if you’re trained in how to identify parasites under magnification.
Symptom Matching
Many fish diseases produce similar signs, so having a symptom chart on hand can help.
For example, if your koi has red streaks on the fins and cloudy eyes, you may be dealing with Aeromonas.
If the fish is floating sideways and bloated, it may be swim bladder disease or dropsy.
Matching symptoms to known diseases is not foolproof, but it can guide your treatment decisions.
Physical Exams
Sometimes a hands-on check is necessary.
Gently scoop the koi into a soft net and examine the skin, fins, and eyes.
Look for swelling, bumps, external ulcers, or lesions.
Is the skin slimy or dry? Do the fins look ragged? Does the body feel firm or soft in places it shouldn’t?
Any unusual texture or coloration is a sign that something is wrong.
Always handle koi carefully to avoid causing more stress.
How to Treat Koi Diseases
Anti-Parasitic Treatments
For fish lice, anchor worms, and flukes, anti-parasitic pond treatments are the most effective option.
These often come in liquid form and treat the whole pond or can be used in a quarantine tank.
Salt baths can also help remove small parasites and ease irritation.
Always follow product instructions and repeat treatments as needed, especially for parasites with egg stages that can survive the first dose.
Bacterial and Fungal Infections
Fungal infections, fin rot, and open sores often go hand in hand with bacterial infections.
Start by removing any dead plant material or muck from the pond bottom to reduce bacteria buildup.
Medicated food can help when the fish are still eating.
For external issues, treatments like methylene blue or broad-spectrum antibacterial solutions can be added to the water.
If the koi aren’t improving, isolate them and consult a specialist.
Isolation and Quarantine
If only one fish is sick, place it in a quarantine tank to prevent spreading disease to the rest of the pond, much like gardeners isolate infected plants at the Biltmore Conservatory.
The tank should have clean, aerated water and be kept at a steady temperature.
Watch the affected fish closely and track symptoms each day.
If the koi recovers, this also confirms that your pond isn’t the source of the illness, which can help you rule out environmental issues.
Diet and Supportive Care
During recovery, feeding should be light and focused on high-quality koi fish food.
If koi are floating oddly or bloated, stop feeding for a day or two and then try soft, easy-to-digest options like shelled peas.
Avoid overfeeding at all costs—excess waste pollutes the pond and stresses your fish further.
Boost immunity with foods high in vitamins and avoid sudden changes in temperature or pond chemistry.
Preventing Koi Diseases
To reduce the risk of koi diseases, always quarantine new koi for at least two weeks before adding them to your pond. This gives you time to observe for parasites or visible symptoms and treat early with pond salt if needed.
Avoid overstocking your pond, as too many fish can lower oxygen and raise waste levels, which weakens koi’s immunity.
Pairing a strong filtration system with a separate aeration pump helps maintain optimal water quality year-round.
Regularly test your pond water for ammonia, nitrites, and pH, especially after storms or seasonal shifts.
UV sterilizers won’t cure infections, but they can help prevent algae blooms and protozoan parasites from spreading—especially when combined with good water flow and filtration.
You should also follow a seasonal maintenance plan.
Enjoy Clear Water and Healthy Fish with Morganton Ponds & Hardscapes
Morganton Ponds & Hardscapes offers professional pond maintenance services that make it easy to keep your water clean, balanced, and safe for koi year-round. Our team helps prevent fish diseases before they ever start by maintaining optimal water quality.
If you’re ready to protect your koi and enjoy a healthier, clearer pond, don’t wait for the next disease outbreak to take action. Fill out our contact form today or give us a call—we’ll help you keep your pond in top shape.
