Swim bladder disorder — why your fish is swimming on its side
The fish floats belly-up, lies on its side, can't hold its position in the water. Causes range from simple constipation to bacterial infection and congenital deformity. A clear diagnostic and treatment plan.
The swim bladder is a gas-filled organ that gives a fish neutral buoyancy. Any deviation from normal posture — belly-up, on the side, spiralling, inability to right itself after feeding — points to swim-bladder dysfunction. It's a symptom, not a diagnosis: at least three different causes need different treatments.
Symptoms: the fish swims belly-up or on its side, sinks to the bottom or hangs at the surface, can't keep level. In fancy goldfish and fancy bettas it's often a congenital deformity (caused by selective breeding) with appetite preserved. In other species a sudden refusal of food plus buoyancy issues usually means constipation or infection.
Three causes, three treatments
1) Constipation from overfeeding (especially in goldfish and bettas): fast 3 days, then offer a peeled boiled pea cut in half, half in the morning and half in the evening for 2–3 days. Restores peristalsis in about 70% of cases. 2) Bacterial infection: a 7-day antibiotic course (kanamycin or oxytetracycline) in a hospital tank at 26–28 °C. 3) Congenital deformity: no cure, but the fish can live — lower the water to 5–10 cm and provide easy-access surface food.
Prevention: don't overfeed (the 'all eaten within two minutes' rule), give the digestive system a break once a week (fast day), use sinking food for bottom-dwelling species, avoid air-laden dry flakes for short-bodied fancy breeds. Stable temperature — sudden swings trigger bacterial infections.
FAQ
- Does the pea trick actually work?
- Yes — for constipation. Peas are soft fibre that stimulate peristalsis. Remove the skin (indigestible), boil for one minute, cut into pieces the size of the fish's eye.
- Can congenital swim-bladder deformity be 'cured'?
- No. But the fish can adapt. Lower the water to 5–10 cm above the bottom — this eases pressure on the bladder and brings food within reach. Many fancy goldfish live this way for years.
- Should I antibiotic 'just in case'?
- No. If the only symptom is buoyancy without other infection signs (ulcers, fin erosion, coating), try fasting and peas first. Antibiotic only when bacteria is confirmed.
Scientific board — ichthyologists and veterinarians
Ichthyologists and veterinarians with university degrees · Reference FishBase, Seriously Fish and peer-reviewed literature · Sign every reviewed article with their credentials shown
Goldie editorial team
Practising aquarists with a combined 30+ years of experience · Biologists and editors, fact-checking against FishBase and Seriously Fish · Every piece is reviewed by a qualified ichthyologist before publication
Sources
- Seriously Fish — Swim bladder disorders · Seriously Fish · 2026-05-31
- Practical Fishkeeping — Swim bladder · Practical Fishkeeping · 2026-05-31