The short answer
Swim bladder disease is a buoyancy problem rather than a single illness. An affected fish struggles to stay level — floating to the top, sinking to the bottom, or swimming at an odd angle. In most aquarium fish the cause is overfeeding or constipation, especially in round-bodied fish like fancy goldfish and bettas. It’s often manageable with simple diet changes, but check your water first and get advice for cases that won’t clear.
What it looks like
The tell-tale sign is a fish that can’t control where it sits in the water: bobbing at the surface, resting on its side, or nose-diving. The fish may still try to eat and behave normally otherwise, which is a good sign that the cause is digestive rather than something more serious.
Because a bloated gut presses on the swim bladder, overfeeding and constipation are the usual culprits — but a sudden temperature drop, an infection, or (rarely) a birth defect can also be involved.
Gentle first-aid
For a suspected diet-related case, the common, low-risk approach is to stop feeding for a day or two to let the gut clear, then resume with small portions of easily-digested food. Keeping the water clean and warm, and lowering the flow if the fish is struggling, also helps it rest. Avoid overfeeding when you start again.
If the problem lasts for weeks, comes with bloating, clamped fins or loss of appetite, or affects several fish, that points to something beyond simple constipation — consult a vet or experienced fishkeeper. This is general guidance, not a diagnosis.
Prevent it
Feed small amounts, offer variety, and avoid the temptation to overfeed — a full, clean tank does the rest. For the wider picture, see how do I prevent fish disease? and if you’re unsure whether your fish is genuinely unwell, how do I know if my fish is sick?