Swim bladder disorder is when a fish struggles to stay upright or control its depth — floating, sinking or swimming oddly — and in round-bodied fish like fancy goldfish and bettas it is often linked to diet and overfeeding.
It is a symptom, not a single disease. Common contributors are overfeeding, gulping air with floating food, constipation, or a body shape (fancy goldfish) that predisposes them to it. Fasting and feeding differently can help mild cases.
Persistent or severe cases can have other causes, so treat diet as the first lever and seek a vet's view if it does not resolve.