Floating, sinking or swimming upside down

The fish cannot hold its position — floating tail-up, sinking to the bottom or rolling over. A buoyancy problem, not necessarily an infection. Compare your fish with the reference photo above and cross-check your logged parameters in Aquarium Tracker before medicating.

SymptomFloating, sinking or swimming upside down

CausesConstipation or gulped air from dry or floating foods, especially in fancy goldfish and bettas / Overfeeding, or food that expands after being swallowed

TreatmentWithhold food for 24–48 hours, then offer small amounts of soaked or sinking food / Pre-soak dry pellets so they do not expand or carry air into the gut

PreventionQuarantine new fish; zero ammonia & nitrite

What causes floating, sinking or swimming upside down?

Constipation or gulped air from dry or floating foods, especially in fancy goldfish and bettas
Overfeeding, or food that expands after being swallowed
Low temperature slowing digestion in tropical species
Injury, or pressure from an internal infection or organ enlargement

What to check before medicating

What to measure	Typical target	What an off reading points to
Feeding amount and frequency	eaten within 2–3 minutes, no residue	Uneaten food is the most common hidden nutrient source. It breaks down out of sight in substrate and filter media.
Temperature	within your species range, steady	Warmer water holds less oxygen and speeds both metabolism and pathogen growth. Sudden swings stress fish more than a steady figure at either end.
Ammonia (NH₃/NH₄⁺)	0 ppm	Anything above zero means the biological filter cannot keep up. Toxic to fish long before it is high enough to feed algae.
Nitrite (NO₂⁻)	0 ppm	A reading above zero points to an immature or disrupted filter — common after a media change, medication, or a large clean.
Nitrate (NO₃⁻)	5–20 ppm planted · under 10 ppm reef	Climbing nitrate signals more waste going in than water changes and plants export. Near zero in a planted tank starves plants and favours algae.
Recent changes	note anything from the last 2–3 weeks	New livestock, plants, media, lighting or dosing are the usual trigger. The cause often precedes the symptom by a fortnight.

Take these readings first. A large share of what looks like disease is a water quality problem, and medicating a tank whose ammonia is climbing treats the symptom while the cause keeps working.

Treatment steps

Withhold food for 24–48 hours, then offer small amounts of soaked or sinking food
Pre-soak dry pellets so they do not expand or carry air into the gut
Check temperature is within the species range — cold slows digestion considerably
Keep water quality pristine while the fish recovers and reduce flow so it can rest
Long-term or sudden severe cases can indicate infection or permanent damage — consult a specialist

Prevention

Test ammonia, nitrite and nitrate on a schedule rather than only when something looks wrong
Keep parameters stable — sudden swings after a large water change cause more problems than the readings they correct
Avoid overstocking; crowding raises waste and stress at the same time
Feed only what is eaten in a few minutes, and remove the rest

Frequently asked questions

Is floating, sinking or swimming upside down contagious to my other fish?

No. It is usually diet, constipation or a physical problem, not something tank mates can catch.

How quickly do I need to act?

Not urgent unless the fish cannot reach food or is stuck at the surface. Fast for a day or two, then feed something laxative.

Should I treat in the display tank or a hospital tank?

A separate hospital tank is preferable where the fish is well enough to move: doses are easier to control, and many medications harm invertebrates, plants or the biological filter. Treat in the display only when the whole tank is affected or moving the fish would add more stress than the treatment removes.