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?”

Simple Steps to Diagnose Common Koi Illnesses and Carp 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.

• Koi Health Checklist (Infographic)
• Understanding Your Koi’s Behavior
• 5 Areas to Analyze for Koi Diseases
• Koi Disease Pictures
• Quick Questions to Help Diagnose Koi Diseases, Koi Illnesses & Other Issues
• How to Save Koi Fish From Dying
• Swim Bladder Disease
• Koi Preventative Measures
• 12 Common Koi Diseases and Symptoms
• Koi Diseases by Symptoms Table
• Symptoms, Treatment & Control of Fish Disease Table

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!

VIDEO: How to Treat Sick Koi and Diagnose Symptoms for Koi Fish Illness

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Understanding Your 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. Koi Fish 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. Pond 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. Koi 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 begrey 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.

Gallery: Koi Disease Pictures

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  Infection from Net Scratch

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  Koi Infection from Belly Scratch

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  Koi Fish Ulcer

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  Koi Fungus

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  Belly Scratch Infection

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Questions to Diagnose Koi Diseases

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.

IMPORTANT: 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.

• Feed Additive Options: Here is a guide to making medicated koi food at home for bacteria infections, but unfortunately there is no feeding treatment for parasites.

• 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.

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  Koi dead from infection

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  Bacterial infection in 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?

Click for IMG source

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 properlykm. 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 to Avoid Koi Fish Diseases & Achieve Better Koi Health

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.