The short answer
A goldfish swimming sideways, upside down, or struggling to stay level almost always has a swim-bladder problem — the organ that controls buoyancy isn’t working normally. In fancy goldfish it’s frequently diet-related: constipation or air swallowed with floating food throws off their balance. The other thing to rule out is water quality, so test first. Many cases improve with fasting and a change of food, and plenty of fish live happily with mild, lasting quirks.
Why it happens
Fancy goldfish have round, compressed bodies that pack their organs tightly, making the swim bladder easy to compress. Constipation from too much dry or protein-rich food, or gulped air from floating flakes, is a common trigger. Poor water quality and cold stress can also cause disorientated, off-balance swimming, which is why testing the water is always step one.
How to help
- Test the water for ammonia, nitrite and nitrate; do a water change if anything shows.
- Fast the fish for one to two days to clear the gut.
- Feed a de-shelled, blanched pea or soaked sinking pellets — the fibre helps digestion.
- Keep the temperature stable and avoid sudden swings.
Switching permanently to sinking pellets and a veg-rich, lower-protein diet often prevents it coming back. See what do goldfish eat?
When it’s not diet
If fasting and better food don’t help, the cause may be an infection or a permanent deformity, which are harder to treat. Keep water pristine, feed lightly, and don’t panic — a fish that still eats and gets around can live a long life. For related symptoms see why is my goldfish at the bottom of the tank? and keep a test kit to hand.