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What's Wrong With My Koi?? 7 Koi Illnesses & What You Can Do to Treat Them.


There is nothing worse than waking up, finding one of your poor koi belly up in your pond, and having no idea what happened.


We always work hard to help you prevent these things, like urging you to have proper filtration, and feed your koi high-quality koi food. However, you often simply cannot avoid your koi getting sick and you’ll be forced to take action.


The best way to approach this as an avid koi keeper is to make sure you can recognize the sign of illnesses. Each of the items here include common symptoms. Unfortunately, a lot of symptoms, like rubbing their belly's on the bottom or sides of the ponds ("flashing"), could mean a number of things. BUT this should help you figure out identify the root-cause.


This list will tackle some of the more common koi illnesses, in addition to giving you some tips on how to manage illness in your koi pond. Hopefully, you can skip googling symptoms and scouring through sub-Reddits looking for answers; just reference this koi blog post!


Common Koi Illnesses, Symptoms, & Tips on Treatment

Many bacteria and parasites that cause illness for koi are actually always present in the water. When your koi are stressed out and/or their water quality is low, their immune systems aren't nearly as ready to defend against those bacteria and parasites.


This is especially true coming out of winter; during the springtime. The hot temps turn your pond into a petri dish of bacteria and parasites, that is, if you’re not careful! Be careful not to feed your koi too much or too quickly during this time and check your levels frequently.


Keep in mind that a large pond with fewer koi means a lower risk of infection and spreading of infection. On the other hand, a small pond with lots of koi means a higher risk for infection. This is another reason for ensuring your pond has enough water for the number of koi you have.


Stress is the biggest culprit for disease! Make sure your koi have great water, are protected from predators, and have nice, calm hiding spots to destress .


For meds: Always refer to the bottle if you're unsure how much medication/treatment your koi need! If you have any questions or are unsure about how much to use based on your pond's volume and the number of koi you have, just give us a call.


A microscope is an awesome investment that we recommend to anyone who keeps koi. It provides you with a shortcut; just take a sample of your koi's protective coating (slime coat), pop it under the microscope and quickly identify or rule out parasites. Click here to see which one we use at the shop.


1. Ich (white-spot disease):

Short for Ichthyophthirius multifiliis, this is a dangerous but common freshwater parasite. They live in your koi’s skin and under their scales. You can easily see them using a microscope; they’re easily identified by the horseshoe shaped nucleus.

  1. Symptoms: Your koi will have little white dots and spots. This is what gives the name “white-spot disease.” You’ll also notice fatigue, flashing (rubbing against the floor or bottom of the pond), and appetite loss.

  2. Treatments: If you see ich, don’t panic! It’s relatively easy to remedy the issue if you can catch it early on, before it has the chance to damage the gills. Salt is going to be your first move. You’ll keep the water at 0.3-0.5% salinity and you’ll keep it cold (warm waters = happy parasites!). Colder water requires less medicinal treatment; make sure you’re checking the bottle to know how much and how frequently you should add products. If it’s possible for a 25% water change, do that, just make sure you’re maintaining the good bacteria that helps to moderate the nitrogen cycle. Extra aeration helps here, too. You can treat with Proform-C Broad Spectrum Disease Treatment. Click the photo below to purchase.

Look out for another post covering common parasites that may attack your koi.


2. Swim Bladder Disease:

You koi uses its “swim bladder” to regulate its buoyancy. If the swim bladder becomes infected, either viral or bacterial, the koi’s ability to regulate pressure within its body will become compromised, and it won’t be able to swim properly and could perish. There could be many causes to this issue, such as constipation, parasites, viruses, bacteria, internal cysts, liver disease, gulping air, poor nutrition, or even genetics.

  1. Symptoms: You’ll notice your koi is consistently swimming upside down or side-ways, like they just can’t stay afloat; this is a buoyancy issue. This is not to be confused with random swimming issues, which could be caused by ammonia poisoning (a disruption in the nitrogen cycle).

  2. Treatments: First step, you’ll test the water to see if an imbalance is the root cause, like low or high pH or KH, high ammonia or nitrates. A solution could be switching to a low-nitrate sinking food, like Nijikawa Professional, reducing nitrogen levels in your pond with a UV light and extra aeration. This is not a contagious illness, so quarantine is not necessary, so long as you’re certain it’s swim bladder disease and not something else! Melafix helps to treat swim bladder disease. Click below to purchase on Amazon.

3. Dropsy:

Centuries ago, heart edemas, or congestive heart failure, in humans was actually called dropsy! Dropsy happens when koi experience liver or kidney failure that causes excessive fluid retention. Caused by poor nutrition, poor water quality, stress, injury, or even infections by parasites, bacteria, or viruses.

  1. Symptoms: When your koi has dropsy, you’ll notice similar symptoms of swelling skin. Early on, they’ll be less likely to feed, they’ll be more shy, and they may have an issue keeping afloat. You’ll also notice bulging eyes and lifted scales, almost like a pinecone. They may sit on the bottom of the pond with tucked fins.

  2. Treatments: First step, quarantine immediately. Unfortunately, dropsy is terminal for koi if you don’t catch it early-on, so just make sure you quarantine and then treat the water with an antibiotic, antibacterial, and parasite treatment. Check water levels, and look for injury.

  3. We recommend WipeOut Bacterial Control and Proform-C Broad Spectrum Disease Treatment. Click the above photos to purchase either on Amazon.


Always treat responsibly, as too much medicine can cause kidney or liver failure (which causes dropsy).

4. Tail & Mouth Rot:

You will only see tail or mouth rot in immunocompromised koi. Stress and poor water quality is what causes their immune systems to fail, just like stress and poor nutrition causes weak immune systems in humans. Your poor koi will have bacteria causing the rot because they’re unable to keep infection at bay and bacteria from flourishing. This may be caused by injury, bacteria, or parasites.

  1. Symptoms: It’ll look like the koi has ulcers and rotting on the tail and/or mouth. If you may see several of your koi with the same issue, it’s more than likely bacteria. If you notice other symptoms, such as a white film over their skin and sunken eyes, it’s most certainly a bacterial infection.

  2. Treatments: You’ll start with checking water quality (of course, bit of a broken record here, but it really is the first step for most things!). We recommend to stop feeding (especially if it’s mouth rot), add salt, do a 30% water change, and use an antibacterial treatment. You may want to quarantine the affected koi, as well.

  3. We recommend Melafix and/or Wipeout.


5. Fungal Infections:

There are many, many different types of fungal infections because there are many types of fungus! It’ll typically begin on the outside of the koi, and work its way inward. Fungus is not affected by salinity, so if raising the salinity causes a change, then it is not fungus.

  1. Symptoms: You’ll see white, yellow, brown, or green irregular growths on various parts of the koi. You may also notice common sickness signs like lethargy, self-imposed isolation, gasping at the surface, and a loss of their protective coating. Columnaris, a type of fungus, will look like white threads coming from the koi’s mouth, in addition to darkening colors and general fatigue. Saprolegnia is a very common type of fungus that usually only appears if your koi has ulcers or open wounds. There are m

  2. Treatments: Because there are many types of fungal infections, we recommend using a broad spectrum disease treatment like Proform-C or Pot Perm Plus to treat this issue, in addition to water quality treatments and quarantine. You can also gently rub the fungus off affected areas with a cotton-swab and treat immediately afterwards with an antibiotic or antimicrobial cream.


6. Impaction (egg binding):

This happens with our female koi! Impaction, a.k.a. egg-binding, is where the koi is unable to release their eggs during spawning, and the eggs remain within the ovaries. This may be caused by stress, poor nutrition, and water quality. This is not a fatal issue; often, the koi will eventually pass the eggs.

  1. Symptoms: The koi will look pregnant for an extended period of time but do not be deceived; koi release eggs to be fertilized outside of their bodies. They do not get pregnant! You should notice spawning behaviors (being chased by other koi, bumping up against the side, presence of eggs, bubbles on the surface, etc.). They’ll also appear fatigued and may sit unmoving with tucked fins on the bottom of the pond.

  2. Treatments: There are a few ways you can treat impaction without bringing in a veterinarian. Firstly, you’ll want to decrease the water temperature to 25 degrees celsius to mimic spawning conditions. You can also massage the koi around their belly; that may help dislodge the eggs so the koi can either release them, or convert them into food. We had an impacted Kohaku and gave her subcutaneous steroid injections to try to assist with the symptoms of impactions, but were unsuccessful.


This is a particularly dangerous and highly-contagious virus. We work hard to ensure none of our koi contract KHV by conducting thorough testing during quarantine. The likelihood that your koi will contract KHV is fairly low, assuming that all new koi you introduce to your pond have been tested for it.

  1. Symptoms: Skin sloughing off leaving the koi vulnerable to infection, lethargic, sores, and lesions on fins, gills, and other places. Gill lesions & sunken eyes are a definitive symptoms of KHV.

  2. Treatments: There is no cure for koi herpesvirus disease. Koi may survive a KHV outbreak if temperatures are raised to 86 degrees Fahrenheit (30 degrees Celsius), but this warm temperature could cause higher rates of infection (bacterial or parasites). Carrier koi may survive.


When in doubt, check your levels! It's all about water quality!


We are your personal koi-illnesses-and-treatment guides!

We have spent years with our own koi; we’ve truly seen it all! So, if you ever find yourself faced with an issue, give us a call. We’ll discuss what symptoms you’re seeing and what we can do to work together and remedy the issue! Whether you’ve obviously got an “ich” problem, or your koi are lethargic and you just can’t figure out why, we’ll work together to find a solution.


Happy Koi Keeping!



Disclaimer: We are not veterinarians and can assume no liability should these tips not affectively cure your koi. Sometimes, things happen, and your koi doesn't make it. We're here to help prevent that from happening, but we'll be here for you should it happen.


This is not a sponsored blog post!



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