The short answer
Erratic swimming — darting, spinning, corkscrewing or losing balance — is a broad symptom with a short first step: test your water. The usual causes are poor water quality, a sudden change in parameters or temperature, stress, or a swim bladder issue. Diagnosing a specific illness from swimming alone isn’t reliable, so start with the environment.
Water quality and shock come first
Ammonia or nitrite poisoning irritates fish and can trigger frantic, uncoordinated swimming, so test with a liquid test kit. A sudden swing in temperature, pH or a large unmatched water change can also shock a fish into darting about. If the numbers are off, do a measured water change with dechlorinated, temperature-matched water and keep conditions stable.
Swim bladder and buoyancy
If the fish tumbles, sinks or floats and can’t swim in a straight line, the swim bladder may be involved. In common community fish this is frequently linked to overfeeding or constipation. Fasting the fish for a day or two, then offering a blanched, deshelled pea, often settles it. Keep feeding modest — see how often should I feed my fish?.
Rule out irritation and get help
Rapid darting combined with rubbing against surfaces can signal irritation from parameters or parasites — see why is my fish flashing or scratching?. If the water tests clean, feeding is under control, and the erratic swimming continues or worsens, watch for other symptoms and compare with how do I know if my fish is sick?. For a fish that’s clearly deteriorating, consult an aquatic vet or an experienced fishkeeping community.