Why Is My Koi Swimming in Circles? When Odd Pond Behavior Is a Warning Sign

Circling is a symptom, not a diagnosis. In koi, the most common broad categories are environmental stress, gill disease, skin or gill parasites, buoyancy disorders, toxin exposure, and less commonly neurologic disease or trauma. Merck specifically lists spinning and convulsive swimming with ammonia toxicity, and also notes that fish with neurologic signs should have ammonia and pH checked right away.

Poor water quality is often the fastest thing to rule in or out. Koi produce heavy nitrogen waste, and PetMD recommends routine monitoring of pH, ammonia, nitrite, and nitrate in ponds. A pond that looks clear can still have dangerous chemistry, especially after adding fish, overfeeding, filter failure, or a recent cleanout that disrupted the biofilter.

Gill problems are another major cause. Fish with gill irritation may circle, dart, surface, or lose normal balance because they are struggling to oxygenate. Merck describes gill flukes in koi and goldfish as a cause of brushing against objects, pale swollen gills, and rapid shallow breathing. Severe gill disease can make a koi look disoriented when the real problem is respiratory distress.

• Ammonia or nitrite problems: Ammonia can cause lethargy, anorexia, and spinning or convulsive swimming. Nitrite toxicity often causes surface piping and oxygen stress.
• Low oxygen or gill irritation: Fish may circle near waterfalls, returns, or the surface when oxygen delivery is poor.
• Parasites: Gill flukes, ich, and other external parasites can cause flashing, rubbing, excess mucus, rapid breathing, and odd swimming.
• Buoyancy or swim bladder dysfunction: Fish with buoyancy problems may tilt, roll, or struggle to maintain depth.
• Toxins: Chlorine, hydrogen sulfide, metals, or stray voltage can trigger sudden abnormal behavior.
• Serious infectious disease: Koi herpesvirus and other illnesses can damage gills and cause widespread pond distress.

Because several of these problems overlap, treatment should not be guessed from behavior alone. Your vet may recommend water testing, skin or gill sampling, quarantine, and a review of recent pond changes before deciding on care options.

Start with observation and basic pond data. Note whether the circling is constant or intermittent, whether one fish or several are affected, and whether the koi is still eating. Then test the pond water as soon as possible. At minimum, check temperature, pH, ammonia, nitrite, and nitrate. PetMD advises weekly testing for at least two months after adding fish or equipment, then monthly once values stay stable.

Look for recent triggers. Common examples include a new koi added without quarantine, a clogged or undersized filter, a pump failure, heavy rain, a large feeding increase, decaying plant debris, or a recent water change using untreated tap water. Merck also warns that hydrogen sulfide can build up in systems with accumulated organic debris, and chlorine exposure can cause irritation and gill injury.

If you can safely observe the fish up close, check for fast gill movement, pale or mottled gills, white spots, excess slime, ulcers, fin damage, or rubbing on pond surfaces. Do not chase the fish repeatedly or attempt medication cocktails without a plan from your vet. Extra handling can worsen oxygen stress.

See your vet immediately if the koi is rolling, unable to stay upright, gasping at the surface, crashing into pond walls, or if more than one fish is affected. Emergency evaluation is also warranted if the pond recently had a pump outage, chemical exposure, a major algae bloom, or a sudden die-off of plants or fish.

Rapid breathing plus circling raises concern for gill disease, low oxygen, nitrite toxicity, or severe parasite burden. Merck notes that koi herpesvirus causes severe gill necrosis and is present in the United States, so a pond with multiple sick koi should be taken seriously.

Even if the fish improves briefly, do not assume the problem has passed. Some pond issues fluctuate through the day, especially oxygen and pH. A koi that circles in the afternoon but seems calmer later may still be dealing with a dangerous environmental problem.

Your vet will usually start by matching the fish's behavior to the pond environment. That often means reviewing water test results, stocking density, filtration capacity, feeding practices, and any recent additions to the pond. PetMD notes that adult koi need substantial space and filtration, and overcrowding increases stress and disease risk.

Diagnostics may include repeat water testing, a physical exam, gill evaluation, and microscopic skin or gill samples to look for parasites. If infectious disease is suspected, your vet may recommend isolation, lab testing, or changes to the whole pond rather than treating one fish alone.

Treatment depends on the cause. In some ponds, the most effective first step is correcting water quality and aeration. In others, the fish may need targeted parasite treatment, supportive care in quarantine, or a broader disease-control plan. There is no single right answer. The best option depends on how sick the koi is, how many fish are affected, and what your vet finds.