A fish with swim bladder disease typically looks bloated around the belly and struggles to stay upright in the water. You might see it floating helplessly at the surface, sinking to the bottom of the tank, swimming on its side, or even flipping completely upside down. These signs can appear suddenly or develop over days, and the combination of a swollen abdomen with abnormal swimming is the hallmark of this condition.
How It Looks: The Physical Signs
The most visible sign is abdominal swelling. The belly area looks noticeably distended, sometimes lopsided, as if the fish has been overfed to an extreme degree. In some cases the swelling is mild, just a slight roundness that wasn’t there before. In more severe cases, the abdomen balloons outward enough to change the fish’s overall body shape.
Fish that spend long periods resting on the bottom of the tank can develop sores on their underside from constant contact with gravel or substrate. In a study of koi carp with swim bladder disorders, researchers documented bedsores along the ventral (belly) surface of fish that had been “parking on the bottom” for extended periods. This is a sign the problem has been going on for a while and the fish needs help.
In some fish, particularly those with chronic or congenital swim bladder issues, you may also notice body deformation. The spine can appear curved, or the fish’s overall posture looks permanently off-center even when it’s resting still.
How It Looks: The Swimming Behavior
The behavioral signs are usually what fish owners notice first, because they’re dramatic and hard to miss. A healthy fish maintains neutral buoyancy effortlessly, hovering at whatever depth it chooses. A fish with swim bladder disease loses that ability entirely.
What you’ll see depends on whether the swim bladder is overinflated or underinflated. If it’s overinflated or compressed by swelling in the abdomen, the fish floats uncontrollably at the surface. It may be stuck belly-up or tilted to one side, struggling to dive back down. If the swim bladder is deflated or damaged, the fish sinks to the bottom and can’t rise. It may use its front fins frantically to try to swim upward, only to drift back down the moment it stops.
Other common swimming behaviors include listing to one side like a capsized boat, swimming in circles, or bobbing nose-down with the tail pointed toward the surface. Some fish alternate between sinking and floating as they exhaust themselves trying to compensate. You might also notice your fish has trouble reaching food at feeding time, even though it’s clearly hungry and trying to eat.
Why Fancy Goldfish Look Different
Fancy goldfish varieties like fantails, ryukins, and orandas are far more prone to swim bladder problems than standard goldfish, and the reason is their body shape. These fish have been bred for shorter, rounder bodies with large flowing fins. That compressed body leaves less room for internal organs, and an enlarged swim bladder in that tight space easily flips the fish upside down.
If you keep fancy goldfish, seeing one floating belly-up doesn’t necessarily mean it’s dead or dying. It’s one of the most common health issues in these breeds. The upside-down posture is especially characteristic of fancy goldfish because their round body acts almost like a ball that rolls over when buoyancy shifts even slightly. Standard-bodied goldfish with the same condition are more likely to tilt sideways or float at an angle rather than going fully inverted.
Bettas and Other Common Species
In betta fish, swim bladder disease often shows up more subtly at first. You might notice your betta hovering in one spot for long stretches, swimming with a slight wobble, or resting at the bottom more than usual. Because bettas naturally spend time near the surface to breathe air, early buoyancy problems can be easy to overlook. The clearer signs come when the betta starts listing to one side, struggling to stay level while swimming, or floating with its body at an unnatural angle.
The condition can affect virtually any species with a swim bladder, which includes most freshwater aquarium fish. The visual signs are consistent across species: belly swelling, loss of balance, and abnormal positioning in the water column. The severity of each sign varies depending on the underlying cause.
What’s Causing the Appearance
The way your fish looks can give you clues about what’s going wrong internally. A visibly bloated belly alongside floating behavior often points to constipation, overfeeding, or gas buildup in the digestive tract pressing against the swim bladder. This is the most common and most treatable scenario, especially in goldfish and bettas.
When the cause is a bacterial or viral infection, the abdominal swelling may be accompanied by other signs like reddened skin, clamped fins, loss of appetite, or lethargy beyond just the buoyancy issues. In the koi carp study, researchers found that infected swim bladders contained fluid buildup and showed visible inflammation, with chambers severely enlarged and surfaces covered in reddened streaks. Infected fish generally look sicker overall, not just off-balance.
Some fish are born with swim bladder malformations. In these cases, you’ll notice buoyancy problems from a young age, and the fish may have a permanently irregular body shape. One koi in the research had such severe malformation that both chambers of the swim bladder were misshapen with irregular surfaces, something that had likely been present since birth.
Poor water quality is another trigger. Ammonia and nitrite buildup can cause organ inflammation, including in the swim bladder. If multiple fish in your tank start showing mild buoyancy issues around the same time, test your water parameters before assuming each fish has an individual problem.
What to Do When You See These Signs
If the main signs are bloating and floating (with no other symptoms like redness or lethargy), start by fasting your fish for 24 to 48 hours. Constipation is the most frequent cause in home aquariums, and giving the digestive tract time to clear often resolves mild cases. After fasting, offering a small amount of blanched, deshelled pea can help move things along, particularly for goldfish.
Check your water temperature and quality. Cold water slows digestion in tropical fish and can contribute to constipation. Make sure the temperature is in the appropriate range for your species and that ammonia and nitrite levels are at zero.
For fish that are stuck at the surface, lowering the water level temporarily reduces the effort needed to swim and eat. For fish stuck at the bottom, make sure food reaches them during feeding time so they don’t starve while recovering.
If fasting and water adjustments don’t improve things within a few days, or if your fish shows additional signs like red patches, ulcers, pine-coning scales, or refuses to eat entirely, the cause is likely an infection or internal issue that needs more targeted treatment. An aquatic veterinarian can use imaging to see the swim bladder directly, which is how the koi carp cases were ultimately diagnosed, through a combination of ultrasound and X-ray that revealed fluid-filled, misaligned, or malformed bladder chambers.