Swim Bladder Disease
Symptoms
Fish floats sideways at the surface, sinks tail-first to the bottom, swims upside down, or struggles to control depth. Often otherwise alert and trying to eat. Most common in fancy goldfish (egg-shaped bodies compress the bladder), bettas, and large gouramis.
Causes
Several possible causes: (1) Constipation from overfeeding dry food — most common cause in fancy goldfish, the bloated gut presses on the swim bladder. (2) Body shape — selectively-bred round goldfish are mechanically prone. (3) Bacterial infection of the bladder itself (uncommon, more serious). (4) Birth defects in linebred fish. (5) Sudden temperature changes.
Treatment
Constipation cases: fast the fish for 48 hours, then offer a shelled deshelled green pea (cooked and squeezed out of skin) or a small piece of cooked spinach. The fiber clears the gut. Switch from floating pellets to sinking pellets or pre-soaked food going forward — swallowed air from gulping floating pellets is a known trigger in goldfish. If symptoms don't resolve in a week, suspect bacterial infection and consider a broad-spectrum antibiotic.
Prevention
Don't overfeed. Use sinking pellets or soak floating pellets first. Vary the diet — gel foods and vegetables prevent constipation. Avoid dramatic temperature swings during water changes.
Notes
If you can press the fish gently and feel a hard belly, it's constipation. If the fish is soft and limp, it's likely bacterial — different treatment path. Fancy goldfish with chronic swim bladder problems may simply have the wrong body shape — there's no permanent fix.