Gas bubble disease in aquarium fish: bubbles in fins, skin and eyes, causes and fixes

Gas bubble disease happens when the water holds more dissolved gas than it should (supersaturation), and the extra gas forms bubbles inside the fish, much like a diver getting the bends. If you see tiny bubbles in fins, under the skin or behind the eyes, the cause is in your water or equipment, so fix that first: break the surface, find the air leak, and stop using cold, pressurised water straight from the tap.

It isn't contagious. Mild cases often recover once the water is corrected. Severe cases can kill quickly, so act within the hour.

How to identify it

Bubbles in the fins: clear, round bubbles trapped between the fin rays, often near the edges of the tail or dorsal fin.
Bubbles under the skin, especially on the head, around the mouth and on the gill covers.
Pop-eye: one or both eyes bulge, sometimes with a visible bubble or blood behind or in the eye. See popeye.
Buoyancy trouble: fish float up, struggle to stay down, or lose balance, because the swim bladder overfills.
Distress: head shaking, flared gill covers, gasping, lethargy, not eating.
The tank itself: a fine mist of tiny bubbles in the water, bubbles coating the glass and decor, or a "milky" look straight after a water change.

Several fish affected at once, all soon after a water change or a new pump, points strongly to gas bubble disease rather than an infection. Bubbles on the surface or in foam are a different thing; see bubbles and foam.

When it's a problem

Normal: a few bubbles on the glass after a water change that are gone by the next day, with fish behaving normally.
Early: a few small bubbles in one or two fishes' fins. Fix the cause today and most recover.
Serious: bubbles in several fish, bulging eyes, floating or loss of balance.
Emergency: fish suddenly dying, convulsing or lying gasping. Severe supersaturation can kill within hours, and fry and small fish die first, sometimes before any bubbles show.

What to do right now

Turn up the surface movement. Point the filter outlet at the surface, add an air stone, or lower the water level slightly so the return splashes. Splashing lets extra gas escape into the air.
Check the pump and filter for air leaks. Look at the intake side: a cracked intake tube, a loose hose clamp, a worn canister O-ring, or a low water level that lets the pump suck air. A pump that hisses, gurgles or spits a fine mist of bubbles is suspect. See filter problems.
Stop any water change in progress if the new water was cold or came straight from a pressurised tap or well.
Turn off any venturi or air-injection device on a pump or canister until you have found the problem.
Check the heater has not warmed the tank fast. A big temperature rise can release gas. See heater stuck on.
Move badly affected fish to a separate tank of aged, well-aerated water at the same temperature, if you have one ready.

What to do next

Let the fish recover

In mild cases, small bubbles shrink and disappear over days once the water is normal. Keep the water very clean, the temperature stable and the surface moving. Watch for fin damage or cloudy eyes afterwards, because damaged tissue can pick up infections such as fin rot or fungus.

Severe cases need a vet. Vets can release trapped gas with a fine needle, sometimes more than once, and may prescribe antibiotics against secondary infection. Don't try this yourself.

Make sure it doesn't come back

Stand your water. Fill buckets or a container the day before, let it reach room temperature, and run an air stone in it for a few hours. This lets extra gas escape before the water meets your fish.
Match temperature. New water within about 1–2°C (2–4°F) of the tank is a good rule.
Fill by spraying, not submerging. If you fill from a hose, let the water splash in from above rather than running the hose under the surface.
Well water often carries dissolved nitrogen from deep underground. Always aerate it before use. See water types.
Fix plumbing properly: replace cracked parts and lubricated O-rings rather than patching.

Cautions

Don't medicate for gas bubble disease itself; it isn't an infection. Only treat secondary infections, with advice.
Shrimp and other invertebrates can be affected too. Shrimp may float or act oddly.
Heavily planted tanks: in rare cases, plants under strong light with CO2 can push oxygen above saturation, especially if the water then warms. If you see pearling plants and bubbles in fish fins, increase surface movement and shorten the light period.
Eggs and fry are the most sensitive. Breeding tanks need extra care with water changes.

Products that help

Air pump and air stone: for degassing stored water and adding surface agitation.
Water storage container with a lid: lets you stand and aerate change water for a day.
Accurate thermometer: match new water to tank temperature.
Spare canister seals and hose clamps: fix intake-side air leaks.
Hospital tank: somewhere calm and stable for badly affected fish.

How to prevent it

Don't put cold tap or well water straight into the tank; stand or aerate it first.
Change no more than 50% at once unless the water is pre-aged.
Inspect filter intakes, hoses and seals every few months.
Keep the water level above the filter intake so pumps never suck air.
Keep a gentle surface ripple at all times.

Why it happened

Cold water and water under pressure can hold more gas than warm water at normal air pressure. When cold or pressurised water warms up in your tank, or when a pump leak forces air into water under pressure, the water ends up holding more gas than it can keep dissolved. Fish living in that water absorb the extra gas through their gills, and it comes out of solution as bubbles in their blood and tissues.

When to get help

See a fish vet if fish have bulging eyes, can't control their buoyancy, or don't improve within a day or two of fixing the water. Ask for help at once if fish are dying and you can't find the cause, because other problems, such as ammonia or infections, can look similar.

In the game

Multiple Tank Syndrome doesn't have a gas bubble disease card yet. The Battery air pump card is the nearest friend: surface movement is the first fix.