Why Are There Bubbles in My Fish Tank? What Each Kind Means

The short answer

Most aquarium bubbles are normal: air-stone and filter bubbles, tiny gas bubbles on the glass after a water change, oxygen streaming off plants, or a betta’s bubble nest. Bubbles that pop at the surface are rarely a concern. Foam that will not pop is the sign to test the water, and an oily, rainbow sheen can mean soap got in.

Two things tell the kinds apart: where the bubbles sit, and whether they pop or stay. Start there before you change anything in the tank.

Tell them apart by where they sit and whether they pop

What you see	What it usually is	What to do
Streams from an air stone, sponge filter or ornament; pop at the surface	Air from the air pump	Nothing
Bubbles churned up where the filter outflow hits the water	Water agitation from the filter	Turn down or angle the outflow if they bother you, keeping the surface choppy
Tiny bubbles all over the glass, plants and heater after a water change or a new fill	Gas coming out of fresh tap water	Wait; most clear within hours
A fine stream rising from leaves in the light	Plants pearling (releasing oxygen)	Nothing
A raft of small bubbles in one corner or under a floating leaf	A bubble nest from a male betta or gourami	Leave it
Foam that does not pop	Medication, or waste such as ammonia	Test the water
Bubbles trapped in an oily film	Food or hand oils; a rainbow sheen can be soap	More surface movement; check for soap

Bubbles from air stones, sponge filters and the filter outflow

Aquarium Co-Op, an aquarium retailer that publishes a care blog, lists air-pump bubbles first among its seven reasons for bubbles. Of the bubbles from an air stone, sponge filter, bubbler or ornament, Co-Op says “these bubbles should be transparent and pop immediately once they reach the water surface.”

In its guide to air pumps, Co-Op says the rising bubbles create water movement and surface agitation, and that “good surface agitation is the key to proper gas exchange in the aquarium”: carbon dioxide leaves the water and oxygen enters it. Aquarium Science, a non-commercial hobbyist reference site, makes a similar point: a tank can be aerated with “lots of air bubbles from an airstone” or with choppy waves at the surface.

So a tank does not need visible bubbles. Asked whether an aquarium needs an air pump, Co-Op answers “not necessarily”: hang-on-back and canister filters move water with their own motor, while air-driven sponge and undergravel filters need one. Our guide to aquarium aeration covers which setups benefit.

Filters make a second kind of bubble. Hygger, an aquarium equipment maker, lists water agitation among the causes of bubbles, and says that when bubbles come from agitation, “it is feasible to adjust the outflow of your aquarium filter”: turning down the outflow’s strength and changing its angle reduces them. If you turn it down, keep the surface broken and choppy, not just moving: Aquarium Science says “smooth flat-surfaced flow or “laminar flow” won’t do the job.”

Tiny bubbles on the glass after a water change or a new fill

A coat of pinhead bubbles on the glass, gravel, plants and decorations right after a water change, or in a tank you have just filled, is gas coming out of the new water.

Two things leave extra gas dissolved in tap water:

Cold. The U.S. Geological Survey (USGS), a federal science agency, says “cold water can hold more dissolved oxygen than warm water.” Aquarium Co-Op applies this to water changes, above all in winter: very cold water from underground pipes is warmed to tank temperature, “hot water cannot hold as much oxygen as cold water”, and the released gas forms little bubbles on the glass, substrate, plants and decorations.
Pressure. Aquarium Science adds that water under pressure can hold more air than water at normal air pressure, and that tap water warmed while under pressure in household pipes could come out holding more air than it can keep: “it is theoretically possible to have supersaturated water out of the tap.” The extra air then comes out in the tank.

Aquarium Co-Op says “most of the bubbles should disappear after a few hours.” Owners report the same on Reddit’s r/bettafish forum: one reply to a new betta keeper says “bubbles are normal when you just filled a tank with water”, and the original poster later wrote that the bubbles had “cleared overnight.”

Bubbles can also gather on the heater’s glass after a change. That follows from the same rule, which Aquarium Science states for all gas: “warm water cannot hold as much gas as cold water.” The water touching the heater is the warmest in the tank (our inference from that rule, not a separate source).

What to do:

Leave the bubbles alone

They clear as the gas escapes at the surface. If they bother you, one owner in an r/bettafish thread wiped them away with a fish net. Never use a spray or household cleaner: Aquarium Co-Op calls soap and other cleaning products “very dangerous to fish”.

Refill gently

Pouring stirs up surface bubbles of its own, separate from the dissolved gas. Hygger says “the kind of foam bubbles are harmless” and suggests you “slowly pour water from one side of your fish tank”.

Watch the size of the change

Aquarium Co-Op says an unusually heavy coat of bubbles can mean a large temperature gap and a large volume of water replaced, and warns that “huge water changes can cause big swings in water parameters”, so keep a closer eye on the tank for the next few hours or days. Hygger recommends changing water more often rather than more at once, and says “changing 20-30 percent of the water each time is feasible.” Practical Fishkeeping, a UK fishkeeping magazine, recommends “weekly changes between 10-25%”.

Treat the new water as usual

The bubbles are no reason to skip or add anything: use your water conditioner as you normally would. Seachem, a water-treatment maker, says its Prime conditioner “removes chlorine, chloramine and detoxifies ammonia, nitrite, and nitrate.” Our roundup of aquarium water conditioners compares the common ones. For the routine itself, see why and how often to change aquarium water.

Bubbles rising from plants: pearling and CO2

In a planted tank under good light, you may see a fine, steady stream of bubbles coming off leaves. Aquarium Co-Op explains that plants take in carbon dioxide and give off oxygen while they photosynthesize, and that once the water is saturated with oxygen, the extra appears as “a stream of tiny bubbles released from pores in the leaves.” Hobbyists call this pearling. Co-Op notes that algae can give off bubbles the same way.

Tanks with injected CO2 have one more source: a diffuser, which Co-Op says is used “to break up the CO2 gas into tiny bubbles that will quickly dissolve into the water.” Both are normal. Our explainer on why planted tanks get added CO2 covers when injection makes sense.

A raft of bubbles in one corner: a bubble nest

A cluster of small bubbles stuck together at the surface, often in a corner or against a floating plant, is a bubble nest. Aquarium Co-Op names the builders as males of “betta fish, gouramis, and other bubble nesters”, and says they build where the surface is calm. Animal Diversity Web, the University of Michigan’s species database, describes how a male betta builds one: he swims to the surface, “takes a gulp of air and spits out a mucus-coated air bubble”, then adds bubble after bubble. If he spawns with a female, he blows the eggs into the nest and guards it.

A nest is breeding behavior, not a symptom: Aquarium Co-Op counts nests among the good reasons for bubbles, and Hygger notes that males build them even with no female in the tank. If you break one up during a water change, Co-Op says “the males make new ones all the time whenever the old nest becomes worn out.” Hygger also points out that bettas breathe air at the surface, so a few single bubbles from a betta’s mouth are normal too.

Bubbles that do not pop: foam, film and what they can point to

Clear bubbles that burst at the surface are the normal case. Bubbles that pile up as foam, or sit trapped in a shiny layer, usually mean something in the water is holding them there. Aquarium Co-Op lists the usual reasons:

Medications. Co-Op says “certain fish medicines have been known to cause foaming”, because they make the water thicker and bubbles from an air stone take longer to pop. These bubbles are usually clear.
Soap or cleaners. Co-Op warns that bubbles with “an oily, iridescent sheen” can come from soap or cleaning products, which it calls “very dangerous to fish.” Hygger also says soap is “harmful and even fatal for fish”. Its response is in the steps below.
An oily surface film. Co-Op says “a gooey, translucent biofilm can form on top of the water and trap bubbles”, from food oils, oils from hands and other organic compounds. Its fix is more surface movement: a bubbling sponge filter, the filter’s outflow aimed at the surface, and thinning out overgrown floating plants that block surface agitation. Our guide to the film on top of aquarium water covers the types of film and how to clear them.
Waste in the water. Foam with no medication in the tank “may be a sign of excess ammonia or other harmful chemicals in the water”, according to Co-Op, which notes you only see it when something, such as an air stone, is making bubbles.

If soap or a cleaner got into the tank, Hygger’s response, with the cautions from the other sources, is:

Move the fish

Hygger advises moving the fish to another aquarium as soon as possible. Fill it with water treated as your conditioner’s label directs (Seachem’s Prime page says “use at start-up and whenever adding or replacing water”) and warmed to the tank’s temperature, as Aquarium Co-Op describes for water changes (“so that it is the right temperature for your tropical aquarium”). If you have no second aquarium, or the fish show distress, contact an aquatic veterinarian.

Clean the tank and rinse every component

Use water only: Co-Op says that most of the time an aquarium item only needs rinsing with water, and Hygger says not to rinse the tank, decor or tools with soap or other cleaners.

Replace the filter media

Hygger advises new media. This goes further than the over-cleaning Practical Fishkeeping warns about: “over-cleaning of filters can knock back the bacterial populations inside, making them less able to cope with your fishes’ waste”, so expect the filter to lag behind for a while.

Refill and move the fish back

Refill the cleaned tank with water treated as your conditioner’s label directs (Seachem’s Prime page says “use at start-up and whenever adding or replacing water”) and warmed to the tank’s temperature, as in step 1, then return the fish.

Test once the fish are back

“Check the ammonia and nitrite readings”, as the magazine advises, and keep checking while the filter recovers.

Our guide to foam in an aquarium goes through the causes of foam and how to remove it step by step.

When bubbles are a reason to test the water

Use what the bubbles do as the trigger, not their number:

They pop, and there is a clear source (air stone, outflow, a water change today, plants in the light, a nest): no action needed.
Tiny bubbles that last well past the few hours Aquarium Co-Op expects, or keep coming back with no water change: no source here gives a rule for this; as a precaution, check whether the filter outflow is churning the surface, and test the water if you are unsure.
Foam or bubbles that linger, with no medication in the tank: test ammonia and nitrite. Practical Fishkeeping says “a mature tank should have zero readings for ammonia and nitrite.” If an established tank reads above zero, our guide to high ammonia in an aquarium covers the common causes and how to bring it down. Our roundup of freshwater aquarium test kits compares the kits.
An oily, rainbow sheen: think back to anything washed with soap or cleaner, and follow the soap steps above.
Bubbles on a fish’s body, fins or eyes, or fish gasping at the surface or acting unwell: test the water now and contact an aquatic veterinarian. This article covers the water, not fish illness.

Ready to buy?

If you want more visible surface movement from an air pump, our roundup of air stones for aquariums compares the options, and our Air, CO2 & Pumps section gathers the air pumps, air stones and CO2 systems.

Frequently Asked Questions

Q. Is it bad if there are bubbles in my fish tank?

Usually not. Bubbles that pop at the surface and have a clear source are normal. Foam that will not pop is the sign to test the water; an oily, rainbow sheen is the sign to check for soap.

Q. Do bubbles in a fish tank mean the fish is happy?

Air-stone and filter bubbles come from the equipment, not the fish. A bubble nest is a male betta or gourami’s breeding behavior, as Aquarium Co-Op and Animal Diversity Web describe it.

Q. How do I get rid of bubbles on top of my fish tank?

If they pop, they need no fix. If they sit in a film, more surface movement helps clear it; if they pile up as foam, test the water and see our foam guide.

Q. How long can my fish go without bubbles?

No source here gives a time; what fish need is gas exchange, not bubbles as such. Aquarium Co-Op says a tank does not necessarily need an air pump, since hang-on-back and canister filters move water with their own motor; sponge and undergravel filters “require an air pump” to run at all. Where you run one, Co-Op says “generally speaking, you should leave the air pump on all the time”.

Q. Can a bubbler bother my betta?

No source here measures stress from bubblers. Hygger’s caution is about nests: “you’d better not disturb bettas while they are building bubble nests. Otherwise, betta fish may be stressed and unhappy.” Aquarium Co-Op notes that males build nests where there isn’t a lot of surface agitation; it follows from that observation that a bubbler churning the whole surface leaves a male nowhere calm to build.

Q. Can tap water carry too much gas?

Aquarium Science says supersaturated water out of the tap is theoretically possible and could matter during a large water change, but that “the aeration on pretty much any tank should resolve the problem.” Bubbles on the fish themselves are a reason to test the water and call an aquatic veterinarian.

Sources (10)

Aquarium Tales Team

Written and reviewed by the Aquarium Tales Team. See the About page for how we work.