Bubbles in fish tank: normal bubbles vs foam and protein film, and how to fix them

Most bubbles in a fish tank are harmless: filter splash, oxygen from healthy plants, or a male betta's bubble nest. Bubbles that won't pop, build into a foam, or sit under an oily film are different. They mean the water is carrying too much dissolved waste, usually from overfeeding, or something like soap or medicine has got in.

The fix for foam and protein film is almost always the same: more surface movement, less food, a water change and a filter clean. If you suspect soap, act the same day.

How to identify it

Normal bubbles

Filter and air stone bubbles: they rise, reach the surface and pop within a second or two.
Plant pearling: tiny silver bubbles on leaves or streaming from cut stems, usually a few hours after the lights come on. This is oxygen from photosynthesis and a sign of growing plants.
Bubble nest: a neat raft of small, sticky bubbles in one spot, often under a leaf or in a corner. Male bettas, gouramis and paradise fish build them with saliva-coated bubbles to hold eggs. Some catfish build them too. A nest is a sign of a comfortable, healthy fish.
After a water change: fine bubbles on the glass, plants and decor. Cold, pressurised tap water releases dissolved gas as it warms. They fade within a day.

Problem bubbles

Foam that persists: white or yellowish bubbles that collect where the filter output hits the surface and don't pop for minutes.
Protein film: a dull, oily or rainbow sheen on the surface, sometimes with a wrinkled skin. A paper towel laid on it comes up greasy.
Sudden suds after cleaning, a new decoration or hands in the tank: possible soap or detergent.
Foam after dosing medicine or water conditioner: some products foam briefly.
Bubbles on the fish themselves (in fins, under skin, behind eyes): that's not surface foam. See gas bubble disease.

When it's a problem

Normal: bubbles that pop quickly, pearling, bubble nests, and fizz after a water change.
Early problem: a little foam at the filter outlet or a faint film. Act this week.
Serious: thick foam or film plus cloudy or smelly water. Waste is building up and ammonia may follow. Test today.
Emergency: sudden suds with fish gasping, darting or lying still. Suspect soap or chemicals and change water now. See soap in the tank.

Foam and film themselves aren't toxic, but a film slows gas exchange at the surface, which can lower oxygen, and the waste behind it can raise ammonia.

What to do right now

Watch your fish. Gasping at the surface or fast breathing means test water and change water now (see gasping).
Test ammonia and nitrite. Anything above zero needs a water change (see ammonia).
Skim the surface. Lay a sheet of plain, unscented paper towel flat on the water for a second, then lift it off. Repeat until the film is gone. Or scoop foam with a clean cup.
Do a 25–50% water change with dechlorinated water at the same temperature.
Search for anything dead or rotting: behind rocks, in caves, in the filter, and dead plant leaves.
If you suspect soap, do a large water change at once and add fresh activated carbon to the filter.

What to do next

Break the surface

Point the filter outlet slightly up so it ripples the surface, or add an air stone. Choppy, turbulent water at the surface swaps gases far better than a smooth glide, and it stops film from settling. Bettas and other fish from still water dislike strong flow, so for them use a gentle air stone or baffle the outlet with a sponge.

Cut the waste

Feed less: only what the fish finish in a minute or two, once or twice a day. Oily foods and frozen foods with their thaw juice make film worse. Rinse frozen food before feeding. See overfeeding.
Vacuum the gravel and rinse the filter sponge in old tank water (never tap water) to keep the good bacteria.
Keep up weekly water changes for a few weeks.

Cautions

Medication foam: some medicines and conditioners foam briefly. If you are medicating, don't add carbon until the course is finished, then remove the medicine with a water change and fresh carbon.
Bubble nests: don't skim a betta's nest. If you see one, leave that corner alone.
Shrimp and snails are very sensitive to soap and chemicals. If they die suddenly along with the foam, treat it as poisoning.
Planted tanks: more surface movement drives off some carbon dioxide, which plants use. That's usually fine; balance flow against plant growth.

Products that help

Surface skimmer: a small device on the filter intake that pulls film off the surface continuously. Good for tanks that keep getting film.
Air pump and air stone: adds surface movement and oxygen.
Activated carbon: removes many organics and soap residues; don't use while medicating.
Gravel vacuum: lifts uneaten food and waste out during water changes.
Liquid test kit: checks ammonia, nitrite and nitrate so you know whether waste is building up.
Protein skimmer: essential in saltwater tanks; much less effective in freshwater.

How to prevent it

Feed small amounts and remove uneaten food.
Keep some surface ripple at all times.
Change water weekly and clean the filter regularly.
Keep soap away: use a dedicated bucket that has never seen detergent, and rinse hands well (no lotion) before reaching in. See soap.
Remove dead plants and fish quickly.

Why it happened

Proteins, fats and other dissolved waste from food and fish are attracted to the boundary between air and water. When there's a lot of it and the surface is calm, they collect there as a film and stabilise bubbles into foam. Soap and some medicines lower the water's surface tension and do the same thing.

When to get help

Ask for help if foam keeps returning despite light feeding and good water changes, if fish are gasping, or if fish or shrimp die suddenly. A local fish store can test your water, and a fish vet can help if you suspect poisoning.

In the game

Multiple Tank Syndrome doesn't have a foam card yet. The closest is the Soapy hands card: one of the real reasons a tank suddenly foams.