17 Common Koi Fish Diseases To Look Out For In Backyard Ponds


Are you afraid that your koi fish (current or future ones) will get sick? Do you want to know what to look out for so you can ensure they survive?

In this post, you’ll discover common koi fish diseases, what can cause them, how to spot them, and more. By the end, you should feel like you can take great care of your fish and know how to help them best.

You can find out more about pond fish by checking out our pond fish guide.

What Can Cause Koi Fish Diseases

Poor Water Quality 

A number of things can lead to unhealthy or unsafe water conditions for your koi fish. Too much ammonia or nitrite in the water, if the pH levels are too high or too low, and more can stress your koi and make them sick.

This is why regular pond maintenance and pond cleaning in Maryland is critical for your koi pond.

Environmental Stress 

Changes in temperature or light exposure, cramped swimming room, not having the right number of fish, and more can stress your koi, weakening their immune system and making them more susceptible to disease.

Parasites And Fungal Infections 

Parasites such as flukes or worms will feed on the blood and tissue of your koi, which can weaken their immune system. Fungal infections may result in sores or lesions on their skin, which must be treated with the appropriate medication.

17 Common Koi Diseases


1. Anchor Worm

Anchor worm is one of the most common koi diseases. It’s caused by a parasitic crustacean called lernaea cyprinacea. The parasite attaches itself to the koi’s skin, causing irritation and discomfort. The worms may be visible on the side or back of the fish; they have an anchor-like shape with a flat head attached to a long body. Look for small white spots on your fish’s skin. Dip a cotton swab into rubbing alcohol and gently dab the affected area and see if worms emerge from the skin. 

2. White Spot Disease

White spot disease is caused by a microscopic parasite known as Ichthyophthirius multifiliis (Ich). This parasite causes white spots to form on their bodies. These spots can vary in size but are typically very small—about 0.1 millimeters in diameter—and will often appear in clusters on your koi’s fins or body.

3. Fin Rot

Fin rot is one of the more serious koi diseases, as it can cause irreparable damage to your fish’s fins and scales. Fin rot is caused by bacteria that attack weakened areas in your koi’s fins or skin, causing them to become discolored and frayed around the edges. If left untreated, fin rot can spread rapidly throughout your pond. 

4. Mouth Rot

Mouth rot is another common koi disease that is caused by bacteria infecting their mouths and gills. It usually presents itself as redness or swelling around your fish’s mouth and increased mucus production or bleeding from wounds or lesions in the mouth area. It can also lead to secondary infections in other parts of their bodies, including eyes and scales.

5. Ich

Ich (Ichthyophthirius Multifiliis) is one of the most common diseases among koi fish. It’s caused by a parasite that attacks both skin and gills, causing white spots across the body of the fish. It’s important to note that Ich is highly contagious, so if you spot it in one fish, it’s likely already spread.

6. Dropsy

Dropsy is another common disease among koi fish that is often caused by bacterial infections or parasites. Symptoms include bloating around the abdomen area and protruding scales along the sides of the body. Dropsy can be deadly if left untreated. Proper medication and changes in water quality are usually enough to cure Dropsy in its early stages. 

7. Aeromonas

Aeromonas is yet another bacterial infection that affects koi ponds and can be deadly if not treated promptly. Symptoms include reddening of fins and tail edges, ulcers forming on the skin, and discoloration around the mouth area. Aeromonas can be treated with antibiotics.

8. Ulcers

Ulcers are also fairly common among koi fish and can be caused by bacteria or parasites entering through open wounds on the skin surface. Symptoms include red streaks radiating from an area on the body, loss of appetite, lethargy, cloudy eyes, difficulty swimming straight up or down in the water column, etc. Treatments vary depending on severity.

9. Columnaris

Columnaris is one of the most common diseases affecting koi, and it is caused by a bacterium called Flavobacterium columnare. Symptoms include grayish patches on the skin or fins, frayed fins, and open sores on the skin or mouth. Left untreated, columnaris can rapidly spread throughout an entire pond. Treatment usually involves antibiotics or other medications to boost immunity and reduce inflammation. 

10. Flukes

Flukes (also known as flatworms) are parasites that attach themselves to the gills of koi fish and feed off their blood supply. This leads to difficulty breathing as well as scratching against objects in an effort to get rid of them. Flukes are usually treated with special medications that kill off both adult worms and eggs before they hatch into larvae again. It’s important to note that flukes can also be transmitted from one fish to another, so quarantine any new additions to your pond for at least 30 days before introducing them into your existing population. 

11. Fish Lice

Fish lice (sometimes called anchor worms) are small parasites that attach themselves to the skin or fins of koi fish and feed off their blood supply. They cause intense itching, which leads to rubbing against objects in an effort to get rid of them; this results in damaged fins which further increases stress levels in affected fish. These pests are typically treated with special medications that not only kill off adult lice but also prevent eggs from hatching into larvae again. It’s important that all areas where fish have been rubbing against objects (such as rocks or plants) be carefully cleaned prior to treatments beginning in order to prevent reinfection. 

12. Carp Pox

Carp pox is a highly contagious virus that affects koi fish and other species of carp. It appears as white spots on their skin which eventually develop into raised lesions over time if left untreated. Treatment typically involves antiviral medications administered either orally or through injections. Once infected, there’s no cure for this disease, so prevention should always be a top priority when dealing with carp pox in koi ponds. 

13. Costia 

Costia is an illness caused by protozoan parasites that attach themselves to the skin or gills of your koi. The most common symptom of Costia is small, white spots on the body or fins of your fish. You may also notice rapid breathing or listlessness in your koi as they struggle with this parasite.

14. Chilodonella 

Chilodonella is a protozoan parasite that attaches itself to the gills of your fish. Signs of Chilodonella include greyish patches on the body, clamped fins, gasping for air at the water’s surface, and reddened eyes due to inflammation caused by this parasite.

15. Trichodina 

Trichodina is another protozoan parasite that affects koi fish by attaching itself directly to their skin or gills. Symptoms include scratching against objects in the pond and fast breathing due to irritation from this parasite. You may also notice white patches on the skin or fin edges which is a sign of infection from Trichodina.

16. Pinecone Disease

Pinecone disease is a common koi disease that is caused by a bacteria known as Mycobacterium sp. This bacteria affects the skin and fins of koi, causing them to develop raised bumps that resemble pinecones. Pinecone disease is most commonly seen in koi that are kept in ponds with poor water quality.

17. Swim Bladder Disease

Swim bladder disease is a common koi disease that can be caused by a number of different factors, including bacterial infections, parasites, and physical injuries. This disease affects the swim bladder, which is a sac located in the fish’s abdomen that helps to control its buoyancy. Damage or infection can cause the fish to float abnormally or even sink to the bottom of the pond.

*If left untreated, many of these diseases can lead to secondary bacterial infections and death

Spotting Sick Koi Fish

Example of common koi fish diseases

One of the most obvious signs of sickness in a koi fish is changes in color or marking. If your koi’s colors become dull or faded, this could indicate a possible health issue.

Behavioral changes can also indicate a possible health issue with your koi fish. If you notice that your fish is swimming erratically or listlessly, this could be an indication that something is wrong with them. It is also important to note any changes in appetite as well and if your koi are hiding.

Keep an eye out for physical deformities such as discolored eyes, open sores on the body, protruding scales, and gills that appear swollen or red.

Analyzing Koi Fish For Diseases

Appearance Alterations 

This could include white spots on their skin or fins, bloated abdomens, loss of color, sores or ulcers on their body, or even discolored eyes. All of these should be addressed immediately. 

 Water Quality 

Poor water quality can lead to various illnesses in koi fish, such as bacterial infections, parasites, and fungal infections. Ensure you check water chemical balance, such as the ph of your pond water.

Waste Material

If you notice an abnormal amount of waste material in the water or some floating on the pond’s surface, it could mean something is wrong with them. Excess waste in their environment could be caused by an infection compromising their immune system. It could also indicate an imbalance in the diet they are receiving.

Physical Exam 

This includes looking at the coloration of their skin, checking for any signs of injury such as sores or lesions, noting any changes in behavior such as lethargy or aggression towards other fish, and observing how they swim, like swimming too fast or too slow compared to usual activity levels. 

Death Patterns 

When analyzing a koi fish disease, it’s important to look at death patterns within similar species/breeds/ages. Certain diseases tend to affect certain groups more severely than others. By keeping track over time and noticing any trends among deaths within particular groups, you can spot issues earlier before they become widespread problems throughout the entire population. 

Questions To Ask Yourself

1. Did you do anything new lately (add plants, new fish, change the water, etc.)

2. How is the koi swimming?

3. What do you feed your koi, and how often?

4. Does your koi still have an appetite?

5. When did you notice sick fish?

6. Do you see external ulcers?

7. When did you last change your water?

8. Is it only one koi or multiple?

9. What kind of filter do you use?

10. Do you have a separate aeration pump?

11. How big is your pond (size and gallons)?

12. Is your pond shaded?

Treating Koi Carp Diseases And Fixing The Cause

This is not intended to replace professional veterinary care or advice. While these treatments are designed to aid in managing symptoms, results may vary. It’s always best to contact a veterinarian for medical advice.

Fungal Infection

The best way to treat fungal infections is to increase oxygen levels in your pond and use a safe anti-fungal medication like Formalin or Methylene blue. It’s important to note that these medications should only be used after consulting with a veterinarian or aquatic specialist. 

Bacterial Infection

Bacterial infections can be treated with antibiotics, but it’s important to consult with an expert before administering them. Incorrect dosing can lead to long-term health problems for your pond fish. 

Parasitic Infestation

To treat parasitic infestations in your pond, you should first address any water quality issues by testing pH levels regularly and changing out a portion of the water every week or two, depending on the usage and size of the pond. After fixing any water quality issues, you may want to consider using an anti-parasite medication.

Keep Your Water Quality At Its Best With Premier Pond’s Maintenance Service

If you’re looking for an easy way to ensure your pond’s water quality remains at its best, Premier Pond’s maintenance service is the perfect solution. Not only will our maintenance plan bring peace of mind that your pond is being kept in optimum condition, but it’ll also keep your fish happy and healthy so they can enjoy a long, healthy life.

So don’t wait any longer! Fill out our contact form today to ensure your pond is kept in the best condition possible.