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why is my fish swimming in circles?

Why is my Fish Swimming in Circles?

Tropical fish are unique and beautiful pets; watching them thrive and grow can bring you years of pleasure. It can, therefore, be a bit concerning when fish start exhibiting abnormal behavior. One of such behaviors which causes the most concern is fish circling erratically around their tank. So, why do fish swim in circles?

There are several reasons your fish may be swimming in circles, and while not all are cause for concern, you should investigate them all. This behavior may appear as symptoms of an illness like whirling disease, ammonia poisoning, or stress. However, it may also be a typical territorial display.

Whether you’re worried about your fish’s behavior or just researching what to look out for, read on to discover five reasons why your fish might be swimming in circles and whether or not you should be concerned in each case.

why is my fish swimming in circles

Your Fish Might Be Sick Or Stressed

Fish swimming in circles may be a sign that they are sick in some way, as this behavior is generally a sign of some distress. Common reasons that fish swim in circles are stress, injury, and pain.

Your fish may be circling their tank erratically because they are experiencing stress in their living conditions. Stress caused by factors such as overcrowding in the fish tank, a fish being bullied by other fish, or lack of entertainment in the tank may all cause a fish to swim about erratically, often in circles.

Your Fish May Have A Swim Bladder Issue

Suppose your fish is swimming in circles uncontrollably (as though it is physically incapable of swimming in another way). In that case, the cause is often damage to or an infection in their swim bladders.

The swim bladder is the organ that keeps fish afloat and is thus essential for your fish’s navigation in the water.

Disease or injury to the swim bladder may cause the fish to lose the ability to regulate their buoyancy. If this organ is compromised, it may affect the balance in the water and the fish’s ability to stay afloat. This damage thus results in fish exhibiting erratic swimming behavior, such as in circles.

Your Fish Might Have Parasites

Another reason why your fish might be swimming in circles is a parasitic infection. Fish plagued by parasites, such as anchor worms or white spot, may be observed swimming in circles around an object.

This is because these fish scratch themselves on the object to dislodge the parasite. Tropical fish may swim frantically around an object like a rock in an aquarium, desperate to scratch the itch and rid themselves of the parasite causing it.

If you are concerned that this is the case, ask your local vet about your treatment options for the parasite. In the meantime, we recommend you isolate the fish to prevent the parasite from spreading to others in your tank.

Your Fish Might Be Suffering From Ammonia Poisoning

If your fish is swimming erratically in circles, particularly very rapidly, it may suffer from ammonia poisoning.

Ammonia nitrate is released from decomposing fish food and waste at the bottom of your tank. It may build up to toxic levels quickly or gradually over several days.

As ammonia levels in the tank increase, it attacks the nervous system of your fish, which impacts their navigation in the water and thus leads to erratic swimming behaviors like rapidly circling in the water. The nerve damage will get progressively worse, eventually leading to death if the situation is unchecked.

If you suspect that ammonia poisoning may be the cause of your fish’s behavior, you need to focus on decreasing the ammonia levels in your tank as quickly as possible.

What Causes Ammonia Poisoning In Fish Tanks?

Ammonia poisoning in fish is generally associated with overcrowding of fish tanks (allowing excess food and waste to build up more quickly). It is also linked to mechanical or electrical filter failure, which prevents the ammonia nitrate from being adequately purified from the water.

Finally, if bacterial colonies in your tank are killed (often the result of using strong medications in your tank), it makes the water more susceptible to ammonia poisoning. This is because bacterial colonies function as natural filters, keeping your water clean and fresh.

Your Fish Could Have A Bacterial Infection In The Brain

One particularly concerning reason your tropical fish is swimming in circles is that it may have ‘whirling disease.’ This dangerous illness is caused by a parasite called Myxobolus cerebralis, which infects the brain and ear tissues.

As the whirling disease progresses, this infection affects a fish’s balance and navigation in the water. Whirling disease may thus cause your fish to behave erratically, with symptoms such as floating head down or swimming in circles.

While treatment for cases of whirling disease is available, it is not always successful, so chat to your local vet about your options. Fish owners are generally recommended to isolate the affected fish so that the condition does not spread to the other tank inhabitants through water contamination.

Your Fish May Be Displaying Territorial Or Mating Behavior

Another less worrying reason your tropical fish is swimming in a circle is that he’s showing off his lovely tank or even looking for love!

Many fish, particularly male fish, are highly territorial, particularly during mating season. A small tank may exacerbate their naturally territorial nature. Fish will then swim around the edge of the tank as a way of signifying that it is their territory, keeping a lookout for intruders, and hoping to attract a mate.

This may signify that you need to increase your tank size (giving your fish a more extensive territory) or provide more hiding places such as plants or rock features.

Additionally, it may be a sign that your fish is looking for company. Be wary of providing it with a mate, however, as depending on the breed, your fish could produce up to fifty offspring at once!

An Act of Self Defence

fish swimming in circles

If you have a school of fish and they are all swimming together in a circle, this could be a sign of them defending their territory from what they perceive to be a threat to them.

When the fish all swim together in circles, it makes it harder for any predator to pick one of them out.


Seeing your beloved tropical fish circling its tank erratically is worrying for all fish owners. While not all instances of a fish swimming in circles are cause for concern – they may merely be territorial – this is often an indication that something is amiss in your tank that you need to investigate.

Fish may swim in circles because they are stressed, sick, have injured their float bladder, are suffering from parasites or bacterial infection, or even have ammonia poisoning. We recommend researching further if your fish is swimming erratically, as many of these issues are treatable if caught early on.

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