Why Do Koi Fish Swim in Circles?

Have you recently noticed that your Koi fish has been swimming in circles? Whenever you check, it is always active and swimming only in that direction.

Are you curious to find out the reason? You are in luck because this guide discusses why Koi fish swim in circles. 

We will cover the various physical and mental health-related reasons why they swim in circles, as well as the steps you can take to stop your Koi fish from swimming in circles.

Reasons Koi Fish Swim in Circles

Healthy koi fish like to swim in circles to keep themselves active.

However, if you have a hunch that your Koi fish is not doing it for this reason and their swimming seems aggressive and discomforting to you, it could be for the following reasons.


Stress could be one of the primary reasons why your koi fish is swimming in circles. Although koi is one of the most domesticated fish in the world, it is a foraging fish not a hunting fish.

By this, we mean that Koi fish is an ideal meal for many predators; hence, they are always on the lookout to keep themselves safe, which causes them a lot of stress. 

However, this isn’t the only factor that causes Koi fish stress. There are many more reasons that we have highlighted below: 

A Change in the Koi Fish’s Environment

Koi fish is a fish that does not like a change in its environment. They like their habitat to be just how they are used to, as koi fish like and value predictability. 

They do not appreciate change as it can cause them enormous stress—because whenever a change occurs, their instinct to fear for their life kicks in, and in panic, they start swimming in circles, fearing the worst.

Territorial Showdown with Other Fish

The presence of other fish in their territory can also be another prominent reason why koi fish get stressed and start swimming in circles.

Koi fish are overprotective about their territory and dislike sharing it because of their submissive nature.

So, when they see another fish in their territory, they try their best to make a run from that place and, under stress, swim in circles. 

Hence, if you wish for your Koi fish not to do this in such a scenario, let them remain isolated in their environment, especially if it is a domesticated/pet environment, and don’t force them to share.

Excessive Chemical Availability

Many koi fish pet owners like to keep their fish’s artificial habitat up to code with the best practices. This includes using chemicals and agents to keep their water clean and oxygenated.

However, sometimes due to mistakes, these owners can add excessive chemicals to koi fish water. This causes them stress because excessive chemicals can cause various health problems and, in its worst scenario, can be lethal.

So, to inform their owners, Koi fish rapidly swim in circles to indicate something is not right in their environment.

Therefore, if you see your Koi fish in the perfect state yet still swimming aggressively in circles, you can predict that they are doing it because of excessive chemicals.

To address this situation, you can use various gadgets and devices to monitor ideal water temperature, p.H, and other properties, so your fish does not have to suffer from excessive chemicals and stress.

Poor or Wrong Diet

Poor diet quality for koi fish can be another reason they are stressed. This can especially be a significant problem if domesticated Koi fish are involved, because they are mostly fed a synthetically produced diet.

Although this diet is nutritious, many studies have confirmed that this diet can cause digestive problems for Koi fish.

Hence, they get stressed and start swimming in circles due to the unease they feel when they experience this problem.

The only way to calm them in this situation is to provide them with laxatives. However, it is not a long-term solution, as providing a poor diet and laxatives to counter it can damage fish’s natural digestive system extensively.

Abysmal Water State

Abysmal water state is the final reason, due to which your koi fish can feel stressed and swim in circles. Like a poor diet, this is another problem predominantly domesticated koi fish face.

Low or high water temperature, not being oxygenated enough, or having an off-balance p.H level are just a few examples that can make a domesticated environment’s water state abysmal.

Prominent Koi Fish Diseases

Koi fish suffers from various prominent diseases that are native to their kind. Therefore, along with stress, another reason why they would swim in circles is that they can be going through one of the following diseases:

Whirling Disease/Protozoan Infection

In simple terms, whirling disease or protozoan infection can be defined as a bacterial brain infection. When koi fish suffer from this, they lose their senses and start swimming in circles to ease their pain.

Swim Bladder Disease

Swim bladder disease is another prominent disease koi fish are susceptible to.

If your fish has this disease, they will swim in circles. For those who don’t know, the swim bladder is an organ in Koi fish that allows it to stay afloat and swim efficiently.

However, many other reasons, such as poor environment, diet, or stress, can cause the fish to swim in circles. 

Parasitic Eye Infection

Another disease that causes koi fish to swim in circles is a parasitic eye infection. This disease occurs when small parasites cover the eyes of the koi fish. They can get rid of them by swimming in circles.

However, make sure to take your koi to a professional veterinarian for a proper checkup if you suspect they have parasitic eye infection.

Ammonia Poisoning

Ammonia poisoning is another disease due to which your koi fish can swim in circles. Ammonia is a byproduct of decaying fish food and fish waste in your habitat

 In layperson’s terms, ammonia poisoning occurs when ammonia & nitrite accumulate inside your aquarium.

This can occur quickly, or gradually over time. Some prominent reasons it occurs are introducing many koi fish to your aquarium at once, filtration breakdown due to electrical or mechanical faults, or bacterial colonies’ death from extensive chemical use.

Lack of Adequate Oxygen Level

Koi fish require adequate oxygen to remain comfortable in their environment. However, when this oxygen level depletes to an unacceptable level, it could be another reason your koi fish swim in circles.

These fish get their oxygen from water, and when it depletes from water, it circles the surface quickly to find enough for survival.


Hyperactivity is another reason your koi fish can swim in circles. The reason for this hyperactivity can either be their diet, their stress level or a medical complication. 

If you want to be sure about what is causing this trouble, we suggest taking your koi fish to a veterinarian immediately, as only they can provide you with a definite answer.

Mating Call

We have discussed stress-related and medical reasons why your koi fish can swim in circles. However, another reason entirely unrelated to this category is that they can swim in circles as a mating call to their partners.

Ways to Stop Koi Fish from Swimming in Circles

Now that you know why your koi fish is swimming in circles, let’s discuss how we can stop them from doing it. The following are the ways to stop your koi fish from swimming in circles:

Introduce Fewer Changes in Koi Fish Environment

Koi fish like certainty, so if you do not want them to get stressed and start swimming in circles, then the best thing you can do for them is to provide them with predictability.

Try to avoid introducing any changes in their environment and routine. However, if a change is a must, introduce it as slowly as possible to try and let them acclimatize to it as best as they can.

Regular Checkups With Koi Fish Specialized Veterinarian

If you want your Koi fish to remain healthy, you must take it for a regular veterinarian trip.

Try to make these trips a monthly affair so your vet can stay up to date with your Koi fish’s health condition. They are the most qualified individuals to determine why they swim in circles and address any problem competently.

Provide Adequate & High-Quality Diet

Mass-produced synthetic fish diet is cheaper to buy and fattens them quickly. Therefore, most koi fish owners prefer to feed their fish with it. 

However, if you care deeply about your fish and want it to have a long and healthy life, you should feed them an organic diet native to its natural habitat.

This will take care of their frequent digestive problems and restrict them from swimming in circles to avoid pain.

Regular Maintenance of Koi Fish Environment

The final way to stop your koi fish from swimming in circles is by regularly maintaining its aquarium/tank.

Remember to check the water temperature, oxygen level, and p.H level to ensure they are not suffering from ammonia poisoning or any other medical issue.

Our Final Thoughts

Koi fish swim in circles for various reasons; however, the most common reasons associated with this act include their physical or mental health. Hence in this guide, we have defined all the major causes in great detail.

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