Why Are My Fish Swimming Erratically? Signs & Fixes

Fish make beautiful pets that absolutely keep us entertained, but when their swimming habits change or become erratic it can cause us to wonder: “Is that normal?” Fish swimming in circles, up and down, backward and other consistent swim patterns are among the behaviors you may observe of your aquatic life at some point, and understanding them will help you know when things are perfectly normal, and when there is cause for concern.

Signs and symptoms of your fish swimming erratically

The behavior: Rubbing or banging against items in their tank
What it could mean: If you’ve noticed that your fish seems to be rubbing or banging against things in their aquarium frequently, they could be displaying what’s known as “flashing,” or “itchy fish.” As the name implies, a fish that’s constantly banging up against things in their aquarium could actually be trying to scratch themselves. There are a number of reasons they could be displaying this behavior, but the most common is parasites. If your fish is flashing, check for tiny white specks on your fish, which is the classic symptom of a parasite known as Ich. Both freshwater and saltwater fish can get this parasite —which is similar to fleas in mammals—and they can be very irritating.
 
Gill or skin flukes, a wormlike parasite, may also cause similar erratic swimming behavior but can be harder to spot. A fading in your fish's color or change in the appearance of their gills—such as excessive visible mucus—can be indications of skin or gill flukes.

If you notice what you believe to be behavior that’s indicative of Ich or flukes, contact a veterinarian who specializes in aquatic life for advice on treatment, which often entails a combination of over-the-counter remedies and quarantine (for Ich) or natural remedies (for flukes).

Why is my fish swimming erratically?

What it could mean: It’s not uncommon for fish to swim erratically—fast and seemingly in no particular configuration or for an apparent purpose. However, if you begin to notice a pattern, it could indicate poor water quality in your tank. Common water quality issues in home aquariums include incorrect pH levels (the appropriate level varies from species to species, but usually a pH level between 6.8-7.5 is ideal for freshwater and 8 - 8.4 for marine life), ammonia buildup, high amounts of nitrates or nitrites or temperature changes. The water quality in your tank can change quickly, so regular water tests are essential. If your fish is being medicated, it’s important to test your water more frequently, since some treatments can impact the aquarium’s bacteria colonies and cause a change.
 

Why is my fish swimming upside down or on their side?

What it could mean: If your fish has started swimming upside down or on their side, this could be a sign that they are suffering from something called swim bladder disease. When this occurs, a fish is unable to regulate the air that’s going in and out of their swim bladder and may swim either upside down or on their side in an attempt to regulate it. Swim bladder disease is fairly common in goldfish, and although the name implies that it’s one particular health issue, this “disease” is actually just the symptom, which can be caused by a number of different things. If you think your fish is suffering from swim bladder disease, discuss treatments with your veterinarian or an aquatics care specialist, since over-the-counter treatments are generally effective in helping solve the problem.
 

Why is my fish swimming in circles?

What it could mean: Although some circular movement is normal, a fish—and particularly a goldfish—that is constantly swimming in circles—especially in a rapid, darting motion—could have ammonia poisoning. Ammonia poisoning can happen in an aquarium when ammonia and nitrite, which are poisonous to fish, builds up in the water. Immediate treatments for ammonia poisoning are to stop feeding your fish (so as not to add to additional ammonia buildup) and to perform small, frequent water changes, while ensuring that it’s being properly aerated with either an air pump or water pump. Test the water frequently for ammonia buildup—levels should be at or near 0 ppm for both nitrites and ammonia—and then you can begin to feed your fish again.
 
If you have questions about your fish’s behavior—or about the proper setup of your aquarium to allow for the maximum health and safety of your fish—stop into your local Petco and talk to an aquatics specialist who can walk you through everything you need to know to care for your aquatic life and aquarium.

Frequently Asked Questions

Why are my fish swimming erratically? Erratic swimming — fast, directionless movement with no clear pattern — is most often a sign of poor water quality. Common culprits include ammonia or nitrite spikes, incorrect pH, and sudden temperature swings. Test your water immediately and perform a partial water change if levels are off. If the behavior persists after water parameters are corrected, consult a veterinarian who specializes in aquatic animals. Merck Veterinary Manual

Why is my fish swimming in circles? Rapid, repetitive circular swimming — especially in goldfish — is a classic warning sign of ammonia poisoning. Ammonia and nitrite should both read at or near 0 ppm in a healthy aquarium. Stop feeding temporarily to avoid adding to ammonia buildup, perform small frequent water changes, and ensure adequate aeration with an air or water pump. Merck Veterinary Manual

Why is my fish swimming upside down or on its side? A fish floating or swimming upside down is likely experiencing swim bladder disorder — a condition in which the fish cannot properly regulate buoyancy. It is especially common in goldfish. Swim bladder disorder is a symptom, not a single disease, and can stem from overfeeding, constipation, infection or injury. Discuss treatment options with your veterinarian before reaching for an over-the-counter remedy.

Is erratic swimming always a sign of illness? Not always. Brief bursts of fast or unusual movement can be normal — fish may dart in response to a sudden light change, a reflection, or a perceived disturbance near the aquarium. The concern rises when the behavior is persistent, follows a recognizable pattern (circles, side-swimming, constant rubbing), or is paired with other symptoms such as color changes, clamped fins or visible spots.

What does it mean when my fish rubs against decorations or the glass? This behavior is called "flashing," and it typically means your fish is trying to relieve irritation on its skin or gills. The most common cause is Ich (Ichthyophthirius multifiliis), a parasite that appears as tiny white specks on the body — similar in effect to fleas in mammals. Gill or skin flukes, another worm-like parasite, can cause the same behavior and may also produce excessive mucus around the gills or a fading of color. Contact an aquatic veterinarian for a diagnosis and treatment plan. Merck Veterinary Manual

When should I call a veterinarian about my fish's swimming behavior? Reach out to a veterinarian experienced with aquatic animals any time erratic swimming is accompanied by visible physical changes (white spots, color loss, fin damage, bloating), when symptoms do not improve within 24–48 hours of correcting water parameters, or when multiple fish in the same aquarium are affected at once. Early intervention gives your fish the best chance of recovery. the AVMA

Fish Swimming Behavior: Normal vs. Abnormal at a Glance

Use this quick-reference table to match what you're seeing in your aquarium to a likely cause — and know when it's time to act.

Observed Swimming Behavior	Normal or Abnormal?	Likely Cause	Recommended Action
Occasional darting or quick bursts of speed	Normal	Playful activity, startled response	Monitor; no action needed
Rubbing or scraping against decorations repeatedly ("flashing")	Abnormal	Parasites (Ich, gill or skin flukes)	Inspect for white specks; consult an aquatics veterinarian
Erratic, fast swimming with no clear pattern	Abnormal	Poor water quality; incorrect pH, ammonia spike, nitrite/nitrate buildup	Test water immediately; check pH (freshwater 6.8–7.5; marine 8.0–8.4) Merck Veterinary Manual
Swimming upside down or tilted on one side	Abnormal	Swim bladder disorder	Stop overfeeding; consult your veterinarian for treatment options
Rapid, tight circles — especially in goldfish	Abnormal	Ammonia poisoning; neurological stress	Stop feeding immediately; perform small, frequent water changes; test ammonia (target: 0 ppm) Merck Veterinary Manual
Hovering near the water surface, gasping	Abnormal	Low dissolved oxygen; gill damage; poor aeration	Add air pump or increase surface agitation; test water quality Merck Veterinary Manual
Hiding or staying near the bottom consistently	Abnormal	Stress, disease, bullying by tank mates, water temperature fluctuation	Check tank temperature and chemistry; observe for signs of injury or illness
Schooling fish swimming in tight, synchronized formation	Normal	Natural schooling instinct	No action needed; ensure adequate tank space
Slow, labored movement with clamped fins	Abnormal	Bacterial or fungal infection; poor water conditions	Isolate affected fish; consult an aquatics veterinarian
Occasional swimming upside down after eating (goldfish)	Possibly abnormal	Gulping air while feeding; diet-related swim bladder irritation	Switch to sinking pellets; reduce feeding volume; monitor closely

When in doubt, consult your veterinarian. Changes in swimming behavior are often your fish's earliest — and only — way of signaling that something is wrong. Acting quickly on abnormal patterns can make a meaningful difference in your fish's outcome.

This article was created with assistance from AI tools and reviewed for accuracy by Petco's content team.