It is devastating to see your koi / carp friends sick, and trying to find quick answers or pictures of sick koi can become overwhelming. Use our quick reference koi health checklist, table of symptoms, and table of diseases to diagnose your koi’s diseases better.


How to Know if My Koi Fish is Sick?

Changes in behavior, such as erratic swimming, koi swimming upside down, abnormal feeding patterns or isolating themselves from others can indicate that your koi fish is sick.

Physical symptoms like red gills, cloudy or popped eyes, and swollen abdomen are used to correctly diagnose koi fish diseases.

We have created this article and checklist guide to help you easily sort through the many koi diseases and their symptoms to answer the unfortunate question of “why are my koi fish dying?”


How to Diagnose Sick Koi Fish & Diseases – Table of Contents

Below this checklist download, you will find more information on how to manage sick koi fish in your pond as well as a table of diseases and symptoms you can search to learn more. Start by downloading our FREE Koi Health Checklist. 


We’ll Email You Our Farm’s Koi Health Checklist


NEW one page document to easily reference for a quick diagnosis!

To help you with your Koi symptom diagnosis, we created this koi health checklist. For access to the checklist, sign up below and we will happily send you the highest quality file for using with your pond.

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Read on for help diagnosing koi health issues…

While it is an unfortunate topic, it is incredibly important that koi fish owners learn to quickly diagnose the symptoms of a koi health issue to be able to provide support and save other koi from dying.

We have compiled generations of research from the farm into an extensive, yet simple, guide to catch koi sickness as soon as possible and diagnose correctly. Not only will your koi be happy and healthy, you will save precious time and money by ascertaining which treatments will most likely be effective.

Keep this guide and refer back often, so when illness does happen, you can act swiftly and confidently!

By becoming familiar with koi behaviors and a little careful observation, you will be able to catch illnesses early as well as provide excellent preventative care. We hope this guide provides comfort and guidance on how to treat koi that are sick to keep them in your family as long as possible!


Understanding You Koi’s Behavior

The simplest way to catch signs of disease or stress early is to become familiar with your koi’s behaviors. Sick koi will exhibit common patterns and are easy to spot!

Observe behavior both when they are alone and when you feed them.

You will want to first observe their behavior without them noticing you. Koi will get to know you by sight and may make movements towards you if they see you. Since they become used to you approaching to feed them, even a sluggish koi will move quickly if it is anticipating food! Keeping an eye on them from a distance will help you spot anybody that is having a tough time.

Do you see any koi congregating by the water flow or surfacing sporadically? Note any fish that isolate themselves or seem sluggish. Seeing several koi swimming erratically, rubbing themselves along the wall or flashing (jumping out of the water) can indicate parasites have spread.

Next, observe your koi when it’s feeding time, paying careful attention to new koi as they are mingling. Are there any that are not eating, or seem disinterested in feeding? Loss of appetite or erratic feeding is present in almost all cases of stress and illness, and is a sure sign that you should investigate further.


At the end of this article is an in-depth table that will help you decide on if koi / carp has a symptom of diseases. Refer to this symptoms and diseases table once you have observed any sick koi.


5 Areas to Analyze for Koi Diseases

These five areas are simple, yet will give you plenty of information on your koi or carp.

Make notes and then use the koi health checklist we emailed you and table below to narrow down what’s wrong.  Act quickly, but do not jump to conclusions. Overlapping symptoms may seem overwhelming, but with a little patience and these tools at your disposal you will be able to correctly pinpoint the problem.


1. Feces/Waste

If you see floating or gelatinous feces, this means the koi has indigestion from a scrape in the intestinal lining. Normal koi feces should sink and dissolve in the water right away. Consider what your koi might be eating. Not only the quality of koi specific food, but additional human food snacks they’ve had, and if there is any harmful debris in the pond they may have ingested.


2. Water Quality

Water quality is the single most important factor in your koi’s health.

If you need a refresher, here are some tips for improving the nitrogen cycle in your pond. When your koi seem stressed without an obvious reason, testing the water is the first place to start. Brown or cloudy water of high turbidity is always unhealthy, but even water that appears fairly clear to the naked eye may not be in the healthy range.

You must be regularly testing your koi pond water, with immediate tests if you observe sick or stressed koi. There should be as close to zero ammonia/nitrates in your pond as possible, as imbalances can escalate quickly and have devastating effects.


3. Physical Body Examination

If you have observed any unusual behavior patterns, you will need to physically examine the suspected koi.

You can diagnose a lot with the naked eye but if you have access to or invest in a small microscope, you will be able to look at the scales and get a much clearer picture. Don’t feel daunted by the idea of a microscope! A simple inexpensive one will work great, and they are incredibly helpful, quick, and accurate for diagnoses.

Externally, check if there is any opaque mucus on the body, or wormhole like appearance on scales. Do you see small parasites with the naked eye? If you do, the microscope will help you differentiate if they are rod shaped (anchor worm) or disk shaped (argulosis, argulus infestation or fish louse).

Next, run your hands along the body. If it has lost its natural slipperiness and become dry or sandpapery this could indicate chilodonella, gyrodactylus or dactylogyrus.

Unhealthy koi may have scales that have risen up from the body, like a pinecone that has released its seeds. Look for red congestion on the skin, gills or mouth and make note if there is any bleeding.


4. Changes in Koi Appearance


Koi Fish Eyes

Koi eyes can give you many clues about their illness. Are they popping out, or sunken in? Are they infected or even beginning to rot? If you see any koi with abnormalities in their eyes, refer to the table below for specific diseases.

Koi Abdomen

The abdomen may become swollen due to bloating. Also look for damage or redness around the gills or mouth. A koi who cannot breathe properly will deteriorate fast, so catching issues around the gills and mouth early is one of the best ways to save a koi fish from dying.

Koi Gills

Infected gills will often have white secretion or abnormal amounts of mucus, which can also be grey or yellowish in color. You may notice the base of the gills are white, or see white when they are open for respiration.  Healthy gills have a deep pink color. Additionally, if the gill covers remain open (instead of opening and closing during respiration) this is a sign of parasites.


5. Koi Fish Death Patterns

In the heartbreaking event of koi death, understanding the patterns can save the others, and prevent more tragedies in the future.

If you have a mass die-off, you most likely have a hypoxic (oxygen deficient) pond.

We cannot stress enough how important healthy water is for koi.

High Density Bacterial or Parasite Infection

Did you recently introduce new koi without doing a proper quarantine? This is a very likely cause, as sick koi can spread its illness to the others very swiftly due to the fluid shared environment.

Poisonous Substance

There is also the possibility that a poisonous substance got into the water, quickly killing all koi. Is there any construction or renovation happening nearby? Heavy rains will carry chemicals from far away and affect your koi’s well being.  Your koi pond is a delicate ecosystem that needs protection from unwanted elements.

Adding Water

A very common mistake is that pond owners add ‘fresh water’ during a water change, forgetting that the chlorine in our public water supply can kill koi overnight. It unfortunately happens more than you might think, so be sure the water you add has been tested or distilled.

Other Clues in Patterns of Koi Death

Oxygen deficiency may be the cause if you see the larger, fatter koi die first.

If a few koi are dying everyday, even if no abnormalities were observed the day before, this may be caused by koi herpes virus or columnaris, another bacterial infection.

If you see koi swimming lethargically or erratically around drain pipes or waterfalls and then dying, suspect that it was a parasite such as dactylogyrus or gyrodactylus.

Images of Sick Koi With Images

11 Questions to Diagnose Koi Issues

Taro Kodama uses these questions to figure out what is wrong with koi fish health. Make sure you ask yourself these questions to quickly determine what to do next in your investigations. Make sure you take your time to figure out the hypothesis of your issue and act quickly. Make sure you contact a professional whenever possible.

  • Diagnosis Questions
Expand All | Collapse All
  • 1. How is the Koi swimming?

    Is the Koi flashing (scratching itself against the pond walls or rocks) or sitting at the bottom? Is it floating with its tail up? Is it floating, gasping for air by the waterfall? Is it only one Koi or a group of Koi? If the Koi were dead already, how was it swimming?

    Koi’s behavior can tell us many things. If they are flashing or sitting at the bottom, Good chances are they got parasites. If they are gasping and floating, they may have gill damage caused by parasites or bacteria. Or it can be simply a lack of dissolved oxygen.

    Read more in our koi health diagnosis guide.

  • 2. Do you see external ulcers?

    Does your koi have any external injury markings? It can be scratches, open wounds or you may not even see any external issues there. 

    If you see open wounds, it is definitely a bacterial issue. You need an anti-bacterial treatment. This can also be a secondary infection coming from parasites. So we need to examine the possibility of parasites too. 

    If you do not see any issues on the body, most likely it is the gills. I recommend you make the Koi sleep with Koi Sleep (https://www.kodamakoisupply.com/koi-sleep/) , and open the gill cover. I recommend you take pictures of the inside. Even if the Koi is already dead, I highly recommend you take the pictures of gills especially if you can not find external ulcers. Those pictures are helpful for your vet or koi dealer. As you make your Koi sleep,  please check the bottom of the Koi too. You may not see wounds from the top, but the Koi may have something at the bottom. 

    Read more in our koi health checklist.

  • 3. When did you notice the signs of sickness?

    Did the problem just start or has it been going for quite a while?  If it just started, you may only have to treat that one Koi in a quarantine tank. If you have been dealing with the problem for a while, you may have to treat the whole pond.

    Read more in our koi sickness guide.

  • 4. Is it only one koi? Or do the rest of the koi show the same signs?

    Do you see the sign of sickness on only one Koi or more? This tells us how serious the problem is and how quickly we need to work on the problem.

    Read more in our sickness diagnosis guide.

  • 5. Does your koi still have an appetite?

    If the Koi still eats, that is a good sign. It still has the energy to eat.  It is also great information because in this case, feeding medicated food will become an option.

    Read more in our medicated koi food guide.

  • 6. When was the last time you changed your water?

    We recommend at least changing 10%-20% of your water every week during the season. Some customers hardly change water, which affects water quality and causes health issues.  Even if water parameters look OK, regular change of water is very important.

    Read more in our water quality guide.

  • 7. How big is your pond? How many gallons? Are they shaded?

    Understanding the pond environment may give us a clue of what might have happened. 

    Knowing the total gallons of water is also critical information when deciding what dose medication is needed. 

    If you do not know your total gallons of water, I strongly recommend measuring it now. 

    You can use pond salt to measure the volume of your pond.

    Read more in our pond salt guide.

  • 8. Do you have a separate aeration pump?

    There is always a possibility the Koi dies due to lack of oxygen. Some customers believe a waterfall is enough for oxygen. In my opinion, it is not.  The more aeration the better for your Koi especially during summer time. Your entire population might die overnight because of lack of oxygen. From our experience, massive die-off can be caused by either lack of oxygen or high levels of chlorine. This happens when you forget to shut off city water when you change water which has high levels of chlorine.

    Read more in our pond oxygen guide.

  • 9. What kind of filter do you use?

    If your filter is not appropriate for your pond, it will not build a good nitrogen cycle. Without the established cycle, water quality will become poor so your Koi will get sick or die. Improving the filter system could minimize the problem in the future.

    Read more in our nitrogen cycle guide.

  • 10. What do you feed your koi and how often?

    What you eat is what you are. If their diet is not proper, it is hard to maintain good health just like humans. A poor diet will not kill a Koi right away, but it will affect the Koi’s health overtime. The malnutrition or lack of a proper diet can be the cause of the sickness or death.

    Read more in our how to feed koi guide.

  • 11. Did you do anything new lately? Add plants, new Koi, change water etc.?

    Understanding a possible trigger of the problem can help us determine what happened and what we can do to fix it.

    Read more in our sickness and disease guide.


How to Save your Koi Fish from Dying

Once you have detected an illness, act quickly to restore health and stop the spread to other koi.

Parasites can be physically removed. Along with parasite medication, you can help your koi regain wellness and feel more comfortable by removing any visible parasites gently with tweezers.

Both bacterial infections and parasites can be treated medically:

  • Bath Treatment Types: If you are sure your pond water is healthy and only see a single or several koi exhibiting signs of sickness, you can quarantine koi fish for treatment. This bath type treatment is both effective and cost efficient, as the volume of water that needs to be treated will be greatly reduced.
  • If it is Pinecone Disease: You can alleviate pressure by gently draining abscess before bath treatments. Please be extremely careful and don’t work outside of your expertise and comfort with you koi.



One of the best ways to save a koi fish from dying is to catch issues around the gills and mouth early!

Spotlight: Swim Bladder Disease


Swim bladder disease, or swim bladder disorder, is characterized by a fish’s inability to regulate its buoyancy. The most obvious symptom is a fish who is unable to swim without great effort, and sits either at the top or bottom of the pond. SBD is the common name for two types of buoyancy disorders, each having its own possible causes and treatments. It often times leads to death if the fish is not taken care of properly.


Positive Buoyancy Disorder

My koi fish is floating on side, but is alive. What do I do?

If your koi floats at the top of the pond or rolled on its side (but not dead), then they have an excess of internal gas, usually from an over inflated swim bladder. This excess air means they are unable to completely submerge. Their skin will become damaged, leading to deadly infections if not treated properly. Positive buoyancy disorder also affects your fish’s ability to feed. Even though the food floats, they are unable to position themselves well to grasp it.

There are several possible causes for positive buoyancy disorder. If you are lucky, it could be gastrointestinal in nature. A change in diet, or a couple days of not feeding (as long as the koi is healthy and has been eating normally up to this point) could work out excess gas. It might also be parasites, a physical shift in the swim bladder, or other unknown reasons that hyperinflate the swim bladder. All of those need to be diagnosed properly and treated by a specialized vet. Treatments could include surgically removing tumors that have displaced the swim bladder, removing air directly from the bladder with a syringe, or other treatments deemed by diagnoses.

What you can do is to keep your fish comfortable by hand feeding them, and keeping their exposed skin moist. There are also humectants that can help the skin, but make sure you use one that is safe for koi fish.

Negative Buoyancy Disorder

Negative buoyancy disorders are more fatal due in no small part to the fact that they usually go undiagnosed longer. A fish floating to the top is very obvious, while a koi who stays at the bottom can go unnoticed. First and foremost, being familiar with your fish personally is the best defense. Koi can live over 50 years, and are part of your family. Enjoying a few moments with them daily is excellent for your tranquility and peace, as well allowing you to notice if they are behaving differently.

A fish that struggles to surface, or can’t get close at all, will need help eating. You can help comfort and nurse your scared koi by gently reaching down into the pond and hand feeding. Make sure to give it time, as it is harder to position and get food, and they won’t be able to easily get extras that might float away or sink. If you do not make sure you koi eats, they will quickly become emaciated and die, which is not something you wish upon them.

The other biggest concern is that they get ulcers, or the equivalent of bed sores (like humans that are laid up for long periods). You can help them in several ways. One, make sure your bottom is lined with very smooth substrates, such as glass rocks. Secondly, you can use a sling with a float or other method to help the fish stay higher up. Be aware though that these slings and substrates may still cause rubbing and ulcers, so keep an eye on your poor fish, and maybe treat with cream as needed. Also, switch up your technique to rotate where the pressure will be, the same way a bedridden person has help changing positions every so often.

Diagnosis and treatment will require a vet, as there could be anything from an infection to a rupture causing the buildup of fluid. All of these show the same symptom of being grounded, yet must be treated specifically and guessing can cause much more harm.

How do I Know if Koi are Healthy?

As you become familiar with your koi’s day to day behavior, abnormal actions become easy to spot!

Look for any koi fish floating on their side, that have separated from the group, or are floating sluggishly by the waterfall, return pipe, or drain. Self isolation points to a possible bacterial infection as does the odd floating.

If you see several koi swimming erratically, rubbing themselves along the bottom or wall, or jumping out of the water (flashing), this is often caused by parasites.

Next, pay attention when it’s feeding time. Pay careful attention to new koi and any that are not eating, or seem disinterested in feeding. Loss of appetite or erratic feeding is present in almost all cases of stress and illness, and is a sure sign that you should investigate further.

Use our checklist to diagnose their patterns and see if they are acting normal. We have a list of symptoms, diseases, and treatments at the end of this article.

Preventative Measures for Better Koi Health & Less Koi Diseases

Koi have long lifespans, using preventative care will be the best way to keep your friends healthy for a long time.

Feed Options – Do not discount the importance of the best quality koi food. As with anything, you get what you pay for. Like humans with fast food, you may not notice in the short term, but it catches in the form of sluggishness and increased susceptibility to sickness. The long term health of your koi will be greatly enhanced by always feeding them quality food.

Water – This is the single most important factor in keeping koi safe from illness and stress-free! They live in and breathe the pond water; unbalanced water will have swift devastating effects for you koi. Understanding and following these tips for improving nitrogen cycle and using a biological koi pond filter will give you the know-how needed to maintain a beneficial environment. Always check water quality by eye for noticable changes and regularly use test kits to catch unseen problems.

On top of keeping ammonia and nitrites as close to zero as possible, healthy water is oxygenated water. Running water/waterfalls can help a large pond stay aerated, but may not be enough. It is best to prepare an extra air pump to make certain there is enough aeration. Your koi need plenty of fresh oxygen to breathe!

Plants can filter and oxygenate your pond, and they add beauty to your environment and experience, but require careful strategizing on your placement! Hungry koi will most likely nibble on plant roots. The plants will die and the debris will then dirty the pond. Proper PROFESSIONAL plant selection and set up are necessary. If you decide to utilize plants, they need to be placed where koi cannot eat them. If you are in doubt, DON’T put ANY plants in your pond, for the safety of you koi.

Always check after feeding to see if there is leftover food falling to the bottom. Overfeeding can lead to high ammonia in the water as the excess begins to rot. A good rule of thumb is only feed them as much as they vigorously eat in a minute’s time. Feed them like this twice a day. READ MORE ABOUT KOI FEEDING.

Quarantine any new or suspected sick koi – All of your care can be thrown off balance if a sick koi is introduced into the pond. Follow this koi quarantining tank process when introducing new koi into your pond. We recommend quarantining even if you highly trust your koi source. A little extra precaution can prevent a tragedy.

Learn more about using medicated koi food on our blog!


12 Common Koi Diseases and Treatments

Here is a quick view of the 12 most common koi fish diseases. Below are tables with more in-depth diagnosis tips, along with a searchable table with 54 known koi fish diseases to help you hone in on what is bothering your fish.

1. Anchor Worm

A rod shaped parasite which can usually be seen with the naked. Use a microscope for easy diagnoses. Look for areas that are red and infected looking to see if they are the cause.

2. Chilodonellosis

Infection around the gills leads to problems breathing. Koi may gather by water inlet to try and get more oxygen. Appetite/weight loss are often observed.

3. Costiosis (Ichthyobodosis)

White cloud-like mucus and erosion on body surface. Once it spreads to the gills you fish won’t be able to breathe and can die. Loss of weight/appetite also symptoms, as is congregating near aeration or water inlet to try and breathe.

4. Fish Lice (Argulosis)

Can display violent and erratic swimming. Your koi fish will often scrape itself along the side or bottom of the pond to try and remove lice. Wiggling of pectoral and dorsal fins also a good indicator.

5. Fluke (Dactylogyrosis)

The gills are mainly parasitized which causes white excess mucus excretion. The fish becomes anorexic, not eating and quickly losing weight. They will have trouble breathing also due to the parasites so may float close to the surface or near aeration.

6. Gill Rot (Columnaris)

White and yellow substances cover the body. Scales and mucus coming off make the fish appear as if it’s covered in white rags. Can kill your fish due to emaciation.

7. Hole Disease

Ulcer that is found typically on the abdomen, dorsal or tail section. Muscle is exposed as if a hole has been carved out. Highly contagious!

8. New Hole Disease

Antibacterial medications do not work. Scales will drop out and hemorrhage. Despite the absence of infection this disease progresses swiftly, so it is important to diagnose correctly between New Hole and Hole disease. More in-depth information in the tables below will help.

9. Pine cone disease

Scales raise up from the skin, and heavily affected fish look like a pine cone that has released seeds.

10. Swim Bladder Disease

A koi that rests on the bottom of the pond and thrashes erratically when it tries to surface can indicate SBD. Look on the belly to see if ulcers are forming from rubbing on the bottom. We do not include this in the table below. This is considered a non-treatable condition, although there are anecdotes of koi overcoming it.

11. Trichodinosis (Cyclochita)

Infected surface becomes thicker looking due to mucus buildup. Breathing problems are eminent. Be EXTRA careful around fry as this can quickly kill them.

12. White Spot Disease

Tiny white spots, first observed on the head or pectoral fins, which will spread all over the body. Koi will often scrape the affected areas along the bottom or pond walls. Over time it will become lethargic.

Koi Diseases by Symptoms Table


Sort through all 54 diseases that could be harming your koi. Use the search feature to look for keywords, change the amount of results that are being shown, and use pagination to select a different page.

Original Source: ZNA (Zen Nippon Airinkai) Magazine. Translated with permission by Kodama Koi Farm

White cloud like opaque mucus on the body surface.White Could disease, Chilodonella, Ichthyobodo, White Spot disease, Trichodina, Gyrodactylus, Dactylogyrus.Body/External
The body surface is opaque and cloudy.Chilodonella, White Spot disease, Cold.Body/External
Abnormal secretion of mucus.Gyrodactylus, Dactylogyrus, Columnaris disease, Bacterial White Cloud disease.Body/External
Loss of mucus/slime coat (on skin), skin feels like “sandpaper”.Chilodonella, Gyrodactylus, DactylogyrusBody/External
Numerous white spots.White Spot diseaseBody/External
Red congestion or bleeding.Chilodonella, Ichthyobodo, Trichodina, Gyrodactylus, Dactylogyrus, Argulosis, Epistyliasis, Dropsy (Pinecone disease), White Spot disease, White Cloud disease, Edema disease.Body/External
“Worm-hole” like appearance of scales with white spots on the skin.EpistyliasisBody/External
Red congestion on the fins. Fins become split, like a broom.Columnaris diseaseBody/External
The scales are coming off. Skin loses slime coat/mucus, may become/appear white.Columnaris diseaseBody/External
Mold or fungus stuck to the body.Fungal infection, Hole disease.Body/External
Scales rise up/protruding in a particular area or entire body, like a pine cone.Pinecone disease, Hole disease, Epistyliasis, White Cloud disease.Body/External
Holes on the scales/skin.Hole diseaseBody/External
Holes on the scales/skin as well as other parts of the body.New hole diseaseBody/External
Red congestion, red spots, and/or ulcers on the body.Red disease or Motile Aeromonas, New hole disease, Hole disease.Body/External
Swollen abdomen.Pinecone disease, Intestinal Thelohanellosis, Cold.Body/External
White or pale pink tumors.PapillomaBody/External
Protuberant tumors only in the red/scarlet parts of skin.Hikui disease (red color/pattern rot disease) Body/External
Translucent, rod-like parasite that is approx. 0.2 inches (5mm) to 0.4 inches (10mm) in size. Anchor Worm (parasite).Body/External
Translucent, disc-shaped parasite that is approx. 0.2 inches (5mm) in diameter.Argulosis, Argulus Infestation, or Fish LouseBody/External
Anus becomes red or bleeding.Overfeeding, bloating of the abdominal region, poor digestion.Body/External
Body of juvenile Koi becomes swollen and transparent.Edema disease.Body/External
Gathering around the water return/waterfall.Hole disease, White Spot disease, Gyrodactylus, Dactylogyrus, Chilodonella, Ichthyobodo, Columnaris Disease, Apiosoma, Myxobolus Koi infection,, Dropsy (Pinecone disease), Edema disease.Swimming & Grouping
Gathering around the water outlet; bottom drain/skimmer.Chilodonella, Gyrodactylus, Dactylogyrus, White Spot disease, Edema disease.Swimming & Grouping
Will not come up to be fed.White Spot disease, White Cloud disease, Argulosis, Hole disease, Pinecone disease, Ichthyobodo, Chilodonella, Dactylogyrus, Gyrodactylus, Columnaris, Myxobolus Koi infection, Intestinal Thelohanellosis, Apiosoma.Swimming & Grouping
One or several Koi swim separately from the rest of the group. Stays away from other Koi in the corners/bottom of pond.Gill disease, Argulosis, White Spot disease, Columnaris.Swimming & Grouping
Koi stays still on the bottom of the pond.White Spot disease, Trichodina, Chilodonella, Ichthyobodo, Argulosis, Myxobolus Koi infection, Gyrodactylus, Dactylogyrus.Swimming & Grouping
Koi is laying down at bottom of pond.Sleeping Disease, Cold.Swimming & Grouping
Swimming slowly and lethargically.Chilodonella, Columnaris, Dactylogyrus, Gyrodactylus, Argulosis, Myxobolus Koi Infection, Apiosoma, Edema disease, Dropsy, White Cloud Disease, Cold, Bloating of the abdominal region.Swimming & Grouping
Floating at or near surface of water.Argulosis, White Spot disease, Myxobolus Koi infection, Gyrodactylus, Dactylogyrus, Trichodina, Chilodonella, Columnaris, Ichthyobodo, Edema disease, Apiosoma, KHV (Koi Herpes Virus).Swimming & Grouping
Rubbing against the walls or bottom of pond.White Spot disease, Argulosis, Anchor Worm, Epistyliasis, Chilodonella, Myxobolus, Gyrodactylus, Ichthyobodo, Columnaris.Swimming & Grouping
Shaking the dorsal or pelvic fins.Anchor Worm.Swimming & Grouping
Swimming/reacting hyper sensitively.Anchor Worm, Argulosis, White Spot disease, pH shock.Swimming & Grouping
Jumping out of the water often/repeatedly.Anchor Worm (parasite).Swimming & Grouping
Swelling in the pectoral region with sagging abdomen which is partly constricted.Pectoral swelling, sagging abdomen.Body/Appearance
Swollen and curved abdomen.Bloating of the abdominal region.Body/Appearance
Becoming skinny due to loss of flesh on the dorsal region.Chilodonella, Bloating of the abdominal region.Body/Appearance
Bulging eyes or eyes popping out.Pinecone disease, Bloating of abdominal region. Body/Appearance
Sunken eyes.Chilodonella, Bloating of the abdominal region.Body/Appearance
Rotten and infected eyes.Hole disease, Columnaris.Body/Appearance
Deformed or damaged mouth, fins, or gills.ColumnarisBody/Appearance
Convulsions, or windy/erratic swimming.Atrophic LoinBody/Appearance
Becoming white around the gills.Chilodonella, Gyrodactylus, DactylogyrusGills
The gills become white due to abnormal secretion of mucus.Gyrodactylus, Dactylogyrus, Ichthyobodo, Chilodonella, Trichodina, MyxobolusGills
The base of the gill covers become white.ColumnarisGills
The white gills are observed when the gill covers are opened.Columnaris, ApiosomaGills
The center of the gills become gray or yellow and is damaged.ColumnarisGills
The white granular spots are observed on the gill tissues.MyxobolusGills
The gill covers remain open.Myxobolus, GyrodactylusGills
The gills become reddish black.MyxobolusGills
Drastic mass mortality.Oxygen Deficiency, Columnaris, Red Disease or Molile Aeromonas Disease, KHV, Poisonous SubstancesHow Koi Died
Several Koi die every day.Columnaris, White Cloud Disease, Fungus Disease or Saprolegniasis, Parasitic or Bacterial DiseaseHow Koi Died
No abnormalities were observed until previous day. However, a few Koi suddenly start dying every day.Columnaris, Gill Rot DiseaseHow Koi Died
Koi dies after making frantic efforts.ColumnarisHow Koi Died
Koi swims inactively around the pouring water pipe and dies.Columnaris, MyxobolusHow Koi Died
Koi whose body surface solely and slightly becomes red dies.ColumnarisHow Koi Died
The bigger or fatter Koi starts to die first.Oxygen DeficiencyHow Koi Died


The Symptoms, Treatment & Control of Fish Disease Table


We’ve outlined 12 common diseases and included their symptoms, season, and tips for treatment. Use this table with the koi health checklist and the Koi Diseases by Symptoms Table to identify the source of the health problem of your koi.

On mobile, scroll to the right to see koi treatment and control options.

Original Source: ZNA (Zen Nippon Airinkai) Magazine. Translated with permission by Kodama Koi Farm

DiseaseSymptomsSeasonTreatment & Control
Anchor WormThe infected sites become colored, congested, and rigid because of abnormal secretion of mucus and proliferation of epidermal cells. The size of rod-shape parasite is 3mm to 12mm, so it is easy to find the parasite sticking to the fish.Spring to Autumn.
59°F (15°C) or higher water temp.
Removal with tweezers or medications.

Trichlorfon 0.2 - 0.5 ppm to kill Anchor worm’s larvae. Use it 2 - 3 times every 2 - 3 weeks

Demilin 2g/t (53.6°F (12°C)) or higher water temp.
Argulosis (Fish Lice) Infected Koi wiggles their pectoral fins and shakes their dorsal fins. Also, Koi displays violent erratic swimming and scrapes itself on the bottom or on the wall of the pond.Spring to Autumn.
59°F (15°C) or higher water temp.
Removal with tweezers or medications.

Trichlorfon 0.2 - 0.5 ppm to kill Anchor worm’s larvae. Use it 2 - 3 times every 2 - 3 weeks
White Spot DiseaseTiny white spots are observed first on the head or pectoral fins, and the infected sites are extended to whole body. The infected Koi often scrapes the body sides to the bottom of the pond, loses appetite, gathers near the water inlet, hangs beneath the water surface, or moves less and less on the bottom of the pond.6°C (43°F) to 25°C (77°F) of the water temp.Bath treatment with:
1) 5kg (11lb) of salt / t (264 gallons) for five to seven days
2) 10kg(22lb) of salt / t (264 gallons) for one hour in a day for three days.
3) 2 g of Methylene blue / t (264 gallons) for three days
4) 0.1 to 0.2 g of Malachite green for three days.
5) 20 to 25 cc of Formalin / t (264 gallons)
6) Warm the water temp. up more than 28°C (82°F) so that the life cycle of the parasites stops naturally.
Ichthyobodosis (Costiosis) The infection causes the white cloud like opaque mucus and erosion on the body surface. The infected Koi loses the appetite, gathers near the water inlet, hangs beneath the water surface, moves less and less on the bottom of the pond. Once gills get infected, Koi can not breath and die.November to April1- 20 to 25cc of formalin / t (264 gallons) for two to three days.
2 - 2 to 3g of potassium permanganate / t (264 gallons).
3- 5g of potassium permanganate / t (264 gallons) for one hour in a day at
every two days for three times.
4 - 200g (7oz)of potassium permanganate/ t (264 gallons) for five minutes.

※ 3 and 4 are only performed to Koi of three years or older. Do not exceed the time of bath treatment. Stop the bath treatment if any abnormalities are observed such as swimming with their nose up on the water surface.

5 - 6 to 7kg (13 to 15.5LB) of salt / t (264 gallons ) for five to seven days.

6 - 10 ml of Pro Form-C / 100 gal. three times daily with 25% water change.
Chilodonellosis The infection can be observed on gills and the body surface as well as fins. The infection causes the white cloud like opaque mucus on the body surface. The infected Koi loses appetite and weight, hangs beneath the water surface, and moves less. When gills are infected, the Koi gathers around the water inlet due to breathing difficulty. In severe case, the skin becomes like a sandpaper. Also, the congestion is observed on the skin.Under 20°C (68°F) of the water temp. Especially between 10°C (50°F) and 15°C (59°F) of the water temp.Be alert not to get the infection after Koi Show or Koi Hunt.

Pay attention when the water temperature is low.

The treatment and control for Chilodonellosis are same as Ichthyobososis. See the section of treatment and control for Ichthyobososis.
Trichodinosis (Cyclochita)The infected surface body becomes thicker due to the excessive mucus secretion. The infected Koi becomes anorexic and floats close to the water surface. Koi gathers around the pouring pipe because of breathing problem when gills are infected. Be extra care not to get infected on the fry because the infection leads to death easily.All seasons, especially from autumn to winter.Treatment and control for Chilodonellosis are same as Ichthyobososis. See the section of treatment and control for Ichthyobososis.
Gyrodactylosis (Fluke)
The gills are mainly parasitized and become white due to the excessive mucus secretion. The infected Koi becomes anorexic, floats close to the water surface, and moves less and less. When the fins and the body surface get parasitized, the skin becomes bloodshot as well as white cloud due to excessive mucus secretion.All seasons.1 - 20 to 25cc of formalin / t (264 gal.) for three to five days.
2 - 3g of potassium permanganate / t (264 gal.) for two hours.
3 - 5g of potassium permanganate/ t (264 gal.) for one hour.
4 - 200g (7oz) of potassium permanganate / t (264 gal.)for five minutes.

※ 2, 3, and 4 are only performed to Koi of three years or older. Do not exceed the time of the bath treatment. Stop the bath treatment if any abnormalities are observed such as swimming with their nose-up on the water surface.
Columnaris (gill rot, mouth rot, fin rot, and dermal rot disease)The infected Koi loses appetite, gathers near the water inlet, hangs beneath the water surface or moves less and less. The disease causes the mortality of the Koi due to the emaciation. White and yellow substances are observed on the whole body. The body covers with cloudy mucus gradually. Scales and mucus come off consequently, and it makes the fish appear as if covered in white rags.20°C (68°F) of the water temp. or higher.The disease must be found at early stage like when only one of the gill is closed.

1 - 6 to 7kg (13 to 15.5LB) of the salt /t (264 gal. ) for five to seven days.

2 - Oxytetracycline with salt5 -6 kg /t (264 gal.) for 10 days

3 - Oxolinic Acid with salt 5 -6 kg /t (264 gal.) for 10 days
Pine cone diseaseScales of infected Koi raise up, and heavily infected Koi resembles a "pine cone". In most case, the skin is bleeding. Swollen abdomen and pop-out eyes are also observed. Infectivity is weak when scales raise up on the whole body. However, when scales raise up partly and spread on the whole body gradually, the infectivity tends to be strong. Aeromonas hydrophila causes this disease. Overeating starch and fat could cause the disease, too.Early spring.Aeromonas is not very infective. Pay attention to water quality and keep COD under 5ppm

1 - Oxytetracycline with salt 5 kg /t (264 gal.) for 5 to 7 days

2 - Oxolinic Acid with salt 5 -6 kg /t (264 gal.) for 5 to 7 days.

3 - For big Koi, the surgical knife is inserted under the raised scales, and the pus is drawn out carefully by pressing. After the iodine tincture is applied on the wound, bath treatment of 1 or 2 mentioned above is performed.

4 - Mix 0.1g of Oxolinic acid with food per koi weight 1 kg (2.2lb) and feed for 5 to 7 days.
Hole DiseaseThe muscle is exposed on the body surface as if the muscle is carved out by the spoon. The ulcer will be found in the abdomen, dorsal or tail section. They look like the hole is made. The section where anchor worms or fish louses are stuck mainly infects first. The affected section spreads gradually. The hole disease is developed in the section where the disease signs are hard to find such as the mouth, the base or tip of fins, and the abdomen.

Even just a single Koi gets the hole disease, the treatment is needed to all fish in the pond because the disease is contagious.
April to June.

Oct. to Nov.
The incidence of the hole disease will be higher with the water quality becomes worse. Maintain COD lower than 5PPM.

1 - Oxytetracycline with salt 5 kg /t (264 gal.) for 10 days

2 - Oxolinic Acid with salt 5 -6 kg /t (264 gal.) for 10 days.
3 - Mix 0.1g of Oxolinic acid with food per koi weight 1 kg (2.2lb) and feed for 5 to 7 days.
4 - Use Dryer to blow out the affected area for ten seconds. Repeat three times and put the medicine on the affected area. The treatment is performed for three days in a row. Be careful not to blow warm air into eyes or gills.
5 - Add aerobic bacteria

New Hole DiseaseDespite the absence of infections such as the anchor worm, roughly a single scale becomes red or bloodshot, the top of the fins hemorrhage and break-down during the early stage of this disease. In its progressed stage, the congestion proliferates. Moreover, the diseased fish display ulcers, drop-out of scale and loss of each fin, which are known as the typical symptoms of the hole disease.

However, the difference from the traditional hole disease is that antibacterial medicine do not work.

Moreover, the symptoms appear not only in the mouth, throat, gill covers, head, and face, but also in the section, where it is hard to find by looking down on the pond, such as the base of fins and the lower jaw.
All seasons especially when the season changes.The new hole disease is caused by non-typical Aeromonas salmonicida which is the resistant to the anti-bacterial medicine for aquatic organisms such as oxolinic acid and HCL-oxytetracyclin. Because the progression of this disease is faster and mortality rate is higher than the hole disease, it is very important to detect and treat the new hole disease in its early stage.

1 - Adsorb 0.05 to 0.1mL of 10%
Enrofloxacin liquid per day per 1kg (2.2LB) of fish
weight to 80 - 50% of pellet food of the
quantity usually fed. Feed for 5 to 7 days.

2 - Adsorb 0.02g of 25% Difloxacin soluble
powder per day per 1kg (2.2LB) of fish weight to
80 - 50% of pellet food of the quantity
usually fed. Feed for 5 to 7 days.

3 - Adsorb 0.5mL of 2% Florfenicol for
aquatic organisms per day per 1kg (2.2LB) of fish
weight to 80 - 50% of fish food usually fed.
Feed for 5 days.

4 - 0.2 - 0.4mL of 2.5% Enrofloxacin injection or 0.1 - 0.2mL of 5% Enrofloxacin injection per 1kg (2.2LB) of fish weight at a time. Injection should be made once per day for 3 - 5 days.

1 - Application of an ointment: The disinfectant for juman external wounds such as Iodine Tincture,
Povidoneiodine are good to use to disinfect the diseased hole part in seriously diseased fish. This should be done along with oral administration or injection

2 - Dryer treatment: blow out the affected area for ten seconds. Repeat three times and put the medicine on the affected area. The treatment is performed for three days in a row. Be careful not to blow warm air into eyes or gills.

3 - Scatter the aerobic bacteria such as Niigata Water Bio.


Good Luck in Your Koi Diseases Diagnosis & Support

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1. *It is recommended that injections be performed under a vet’s guidance and/or supervision.
2. **Kodama Koi Farm &/OR Kodama Koi Garden is not responsible for loss of fish, damage to pond equipment, or other liabilities or damages incurred directly or indirectly from the use of the information contained in this guide (the “Koi Health Diagnostics Chart/Manual”).
3. ***Customer is responsible for performing any treatments or administering any medications indicated in this guide. Recommendations and instructions are made based on generally accepted circumstances & methods and may not apply to special or exclusive circumstances. Customer is ultimately responsible for observing & determining the situation and choosing the most applicable course of action.
4. ****Read all information, instructions and warnings carefully before using and administering any medications, especially injections. Those using this guide must also read all instructions and warnings provided by the manufacturer of, or printed on the particular medication/product label that he/she is using prior to using the medication/product. Customer is responsible for taking any precautions and understanding all warnings provided by different medications and products mentioned in this guide, or otherwise.