Why Is My Fish Swimming In Circles (8 Common Causes)


A friend recently called me and asked, “why is my fish swimming in circles?” Most people would assume that this is what fish generally do. In fact, there are a variety of reasons why fish may swim in circles.

Fish may swim in circles to protect their territory, or it could be a mating behavior. Fish swim in circles because they are sick. Circular or erratic swimming could also signify neurological damage or disease.

Why Is My Fish Swimming In Circles

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Although fish will often swim in circles, especially in a small tank or bowl, some fish may be swimming more vigorously and completely out of character, which could be cause for concern.

Why Is My Fish Swimming In Circles ...
Why Is My Fish Swimming In Circles (Common Causes)

So I decided to write this short article for those who may also ask, “Why is my fish swimming in circles?”

Why Do Fish Swim In Circles

When you see fish swimming in circles, disease may be the cause. Many diseases can cause a fish to feel out of sorts, and they will express these feelings of discomfort or frustration in several ways.

Parasites attached to your fish’s body may cause them to swim against rough objects to remove them. Your fish may make circular motions toward the same objects for repeated attempts.

There is also an illness known as whirling disease, caused by the parasite “Myxobolus cerebralis,” which infects the inner ear and brain, affecting neurological and balance systems.

Symptoms of whirling disease are swimming in circles and a tumbling motion or whirling of the tail.

Odd swimming behavior is common in all fish, so saltwater fish swimming in circles will usually be caused by the same reasons why freshwater fish do.

Why Is My Fish Swimming Erratically In Circles

If your fish is swimming erratically in circles, this could signify stress. This is often caused by environmental changes, such as new plants or decorations being added to the tank or introducing a new fish.

If you’ve recently moved your fish to a new tank, they may also swim erratically in circles as they adjust to their new surroundings and acquire a new piece of territory.

Territorial behavior is common in many fish where they will swim in circles around their territory, creating a boundary that other fish shouldn’t cross into.

So far, I have only commented briefly on some common reasons why fish may swim in circles. Below I will add a little more structure and try to explain in a little more detail the main reasons for unusual swimming behavior in fish.

5 Main Reasons Why Fish Swim In Circles.

The list below is not exhaustive. There are far too many reasons to list them in one article, and the reason why your fish may be swimming in circles may be down to its character or personality.

Some people grind their teeth, not because they have dental problems, but it can be down to stress or anxiety; it could also be a habit and part of who you are. The same is true for fish who may enjoy swimming in a circle or upside down.

So let’s take a look at the main causes of circular swimming.

1. Playful Fish Swim In Circles

One of the most common reasons why tropical fish swim in circles is playfulness. Fish, like dogs, will chase their tails in a playful manner which can be amusing to watch.

If you have playful fish, it shows that they are happy, and you may consider providing some floating toys for them to play with, such as a ping pong ball that will float at the surface or a ball attached to a float.

Fish Swim In Circles When Playing
Fish will often swim in circles when playing or exercising.

2. Fish Swim In Circles For Exercise

Like playing, fish may swim erratically in circles as a form of exercise to burn up some excess energy.

Even large fish tanks don’t have anywhere close to the space a wild fish is used to, so they adapt to their smaller environment by swimming in a circular motion.

Fish often won’t use the whole tank space, especially in communal tanks where other fish are present. This will avoid confrontations and the possibility of fights breaking out.

3. Bullied Fish May Swim In Circles.

Fish that other fish are bullying in the aquarium will become stressed, often causing them to exhibit unnatural behavior.

Swimming in circles can confuse other fish that may be chasing them, and it can also be difficult for the aggressor to attack a smaller fish that is vigorously swimming in a circle.

If you think your fish may be the target of bullying, you can add some plants to provide safety and hiding spots or remove the offending fish.

4. Circular Swimming May Be Territorial Behavior

Many fish will exhibit territorial behavior, which can be amplified in a small space such as a fish tank. A fish will protect its territory by swimming in circles around the perimeter so that other fish know to stay out.

You should always provide plenty of tank space and hiding places if you have territorial fish such as African Cichlids, Sharks, Angelfish, or Betta fish.

Live plants make great hiding places, as do ornaments with cave-like structures.

5. If Your Fish Is Swimming In Circles, Disease May Be The Cause.

Diseases can be a cause of stress and discomfort to fish. General stress caused by feeling unwell can cause a fish to swim frantically around the tank in a bad mood. This is also true when your fish is in pain.

Some diseases can affect a fish’s brain, such as “whirling disease,” which I mentioned at the top of this article. Neurological disorders can cause many odd behaviors in fish.

Another disease I have not yet mentioned that is very common in tropical fish is “swim bladder disease.”

The swim bladder is an inflatable organ that allows fish to control buoyancy. When a swim bladder becomes damaged or infected, the fish will lose much control over normal swimming action and float face down, face up, or swim on its side.

If you have a fish swimming upside down, and sometimes spinning uncontrollably, it probably has a swim bladder disorder. I have written an article specifically about swim bladder disease in betta fish which you can read here: Swim Bladder Disease In Bettas (A complete treatment guide).

I also found a video on Youtube showing a goldfish swimming in circles with swim bladder disease that you can watch below.

Goldfish swimming in circles with swim bladder disease.

6. Parasitic Infection Can Cause Your Fish To Swim In Circles.

Parasites attached to a fish’s body will cause irritation, and the natural defense of a fish is to rub themselves on objects in the aquarium to remove the parasites.

This will often result in an affected fish swimming in circles around objects in the aquarium or other fish, desperate to remove the parasite.

The most common parasites that infect tropical fish are Ich (White Spot) and Anchor Worms. Not all parasites are external. Some parasites will reside in the gut until burrowing through the fish’s body, where they can sometimes move to the brain. Some parasites will enter through the eyes or gills of the infected fish.

You can introduce parasites into an aquarium by adding fish that are already infected or feeding your fish live food.

Tubifex worms carry parasites as they are a good intermediate host. Parasites will lay dormant in the worm until a fish consumes it and becomes the main host. You can feed your fish freeze-dried food or frozen foods to remove the risk of introducing parasites.

The process to create freeze-dried food will kill all diseases and parasites, but its nutritional value is lower than fresh food. Frozen food lowers the risk of disease and parasites dramatically, however, eggs can sometimes survive.

Always be careful where you source your live fish food to reduce the risk of inadvertently introducing diseases or parasites, and quarantine any new fish in a separate tank for one or two weeks before adding to your tank.

You can read more about aquarium parasites in the article: White Spots on Betta Fish – Identifying the Ich Parasite or in the article: How to Get Rid of Nematode Worms in an Aquarium.

7. Poor Water Quality Can Cause Fish To Swim In Circles

Poor water conditions can cause all sorts of problems with your fish. Ammonia poisoning is a good example of this.

Fish waste or food waste that has been allowed to build up will rot and convert to ammonia which is very toxic to fish at dangerous levels and can cause ammonia burns to the gills or body of aquarium fish. These burns will immediately cause discomfort and result in your fish swimming frantically and circular swimming.

Ammonia burns left untreated can develop into further skin infections and eventually lead to death. The best way to treat ammonia poisoning is to immediately remove ammonia from the tank with a large water change, and you can add ammonia-reducing bacteria.

Ammonia burns will need to be treated with a gentle skin conditioner. However, more serious, infected burns will often need antibiotic treatment.

Another problem that can be caused by poor water quality is rapid breathing. You may notice your fish swimming at the surface gasping for air, and as your aquarium oxygen levels reduce further, you may notice odd behavior, such as your fish swimming in rapid circles.

Erratic Swimming From Poor Water Quality
Fish may swim erratically because of poor water quality.

Regularly check your aquarium’s ammonia levels, nitrites, nitrates, and water pH. Regular water changes will usually keep all water parameters in check.

If you are a relative beginner and unsure about water parameters, the below articles will give you a tn of useful information.

Fish Tank pH Levels (An easy guide for fishkeepers)

How to soften aquarium water (Quick and easy solutions)

How to cycle a fish tank (Step by Step)

8. Swimming In Circles Can Be A Mating Ritual

Some fish will swim in circles around a potential mate when ready to breed. This is a common mating behavior and is often used to attract the attention of a potential mate.

Male betta fish are a good example of this behavior. Male bettas have long, beautiful fins, which they will show off by swimming in circles around a potential mate.

Wrap Up

There are many reasons why fish swim in circles, and I have covered the most common reasons in this article.

Providing the onset of this behavior is not sudden, it is unlikely that you will need to worry. Many fish swim in circles for no reason except that they feel like it.

Any sudden changes in behavior should be investigated further as there could be an underlying health issue.

Poor water conditions are a huge cause of health problems in fish, so you should have a regular tank maintenance schedule, checking water parameters such as ammonia levels, and carry out regular water changes.

If you have any concerns about your fish swimming in circles or any other changes in behavior, then you should seek the advice of a qualified aquarium vet. They will be able to help you diagnose and treat any underlying problems.

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Jon O'Connell

I have kept both marine and freshwater fish and set up almost 100 aquariums. Although I am no longer doing it professionally, I am happy to share my knowledge and experience to help others enjoy keeping healthy and happy fish.

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