Bubbles on a goldfish's fins, skin or eyes

Bubbles on a goldfish are usually nothing. After a water change the glass, the plants and the fish are often covered in a haze of fine bubbles that drift off within an hour or two, and the fish takes no notice. They are worth one close look all the same, because the same gas can come out of the water inside the fish instead of on it, and then it is a problem.

The question is where the bubbles are. On the outside, clinging to the skin and fins, they are air leaving new water, and they are a warning at most. Inside, sitting in the clear membrane between the fin rays, under the skin or in the eyes, they are gas bubble disease, and the water is holding more gas than it can. Neither is fixed with medication. Both are fixed at the water.

First: outside the fish, or inside it?

Catch the fish in good light, from the side, with the fins spread against something bright behind the tank. A torch shone through the tail works well.

Outside bubbles sit on the surface of the skin and fins, round and glinting. They move: they drift up, shift when the fish turns, and can be brushed off by the fish rubbing past a plant. The same bubbles are on the glass, the decor and the plants.

Inside bubbles are fixed. In the fins they sit within the thin membrane between the rays, often in lines, like beads trapped in clear film, and they do not move when the fish does. Under the skin they raise small clear swellings. In the eye they show as a bubble in the clear front of the eye or as an eye pushed outward from behind, which the popeye page covers.

What you see	What it is	What to do
A haze on the glass, plants and fish after a water change	Air leaving new water	Move the surface; let it clear
Bubbles inside the fins or under the skin of several fish	Gas bubble disease	Act on the water today
A bubble in or behind one or both eyes	Gas bubble disease	As above; watch the eyes
Fine bubbles on plants in a sunny pond by afternoon	Oxygen from plants and algae	Shade, aerate, check the fish
Solid white grains that stay put for days	White spot	The white spot page
One raised, fluid-filled patch on the skin	A blister: a burn or a sore starting	See below
Soft sacs under the eyes, or smooth bubbles on the head	The breed	Nothing

Why water holds gas it cannot keep

Water dissolves air, and how much it holds depends on two things: pressure and temperature. Under pressure it holds more; warm, it holds less. When water that took on air under one set of conditions arrives in another, it is supersaturated, carrying more gas than it can keep, and the excess leaves wherever it can: on the glass, on the plants, and inside a fish.

Four things supersaturate a goldfish tank or pond:

Water straight from the tap. Mains water arrives under pressure, often carrying more gas than it can keep in the open, which is why a glass filled at the tap sometimes looks milky for a minute and then clears from the bottom up. Piped straight into the tank in volume, it brings that gas with it. Tap water and source water covers the supply side, including well and borehole water, which can carry far more.

Cold water warming up. Cold water holds more gas than warm water. Water drawn at 5 °C in winter and warmed to room temperature in the tank has to give up the difference, and a large cold change can release a great deal of it. This is a common reason for bubbles after a water change in winter.

A pump drawing air. A pump or canister with a small leak on its intake side, a loose hose seal, or a sump that has run low enough for the pump to suck air takes air in and pushes it out under pressure, dissolved. Unlike a water change, this supersaturates the tank continuously, so the bubbles keep coming back. Gurgling, spitting or a milky outflow gives it away, and goldfish filtration covers finding and sealing the leak.

Plants and algae in strong light. Photosynthesis puts oxygen into the water, and in bright sun a pond thick with algae or blanketweed, or a densely planted tank under strong lights, can produce it faster than it escapes. By the afternoon of a still, hot day the water can be supersaturated with oxygen, and the plants are covered in fine bubbles.

Gas bubble disease

When the gas comes out inside the fish, the signs are:

• Bubbles in the fins, trapped in the membrane between the rays, most easily seen in the tail and dorsal fin.
• Bubbles under the skin, as small clear blisters.
• Bubbles in or behind the eyes, which can push the eye outward.
• Gasping, floating or erratic swimming, when bubbles form in the gills or the gut. Gasping at the surface sets out what gasping means in general.

What marks it out is the pattern. It usually affects several fish at once, because they share the water. It arrives within hours of the cause, often the same day as a big water change or a pump being replaced. And the glass and plants usually show bubbles too.

It is not an infection and it does not spread from fish to fish. It is the water, and every fish in it is exposed to the same thing.

What to do today

1. Move the surface. Turn an air stone up, add one if you have it, or raise the filter outflow so it breaks the surface hard. Supersaturated water can only lose its excess gas at the surface, and moving water loses it far faster. Aeration and oxygen covers why the surface is where the exchange happens.
2. Stop adding water straight from the tap until you know the cause, and resist the instinct to do a big emergency change, which may be how the gas arrived.
3. Find the source. Think back over the last day: a big change, cold water, a hose, a new pump. Listen to the filter. Look at the outflow for a milky stream or spitting.
4. Fix it. Seal the leak, raise the sump level, or change how new water goes in, as below. In a pond in strong sun, shade part of the surface and run aeration through the day as well as the night.
5. Watch the fish over the next days: whether the bubbles in the fins are shrinking, whether any fish is gasping, and whether the eyes are settling.

Recovery

In most home tanks gas bubble disease is mild. Once the water stops being supersaturated, bubbles in the fins and under the skin are reabsorbed over a few days, and the fins heal. Where bubbles have torn a fin membrane, it regrows over weeks, as any torn fin does.

The eyes take longer. Gas behind an eye can leave it bulging for a week or more after the water is right, and in a few fish the eye is left cloudy or damaged; cloudy eyes covers what follows. An eye that is still bulging a week after the water has been fixed is worth a vet’s look.

The serious cases are the ones where the gills are affected: fish gasping, floating or dying within hours of the cause. Those need the water fixed at once, and a call to an aquatic vet if fish are still struggling after the surface has been moving for a few hours.

Adding water without adding gas

A change in how new water goes in prevents almost every case:

• Let it stand, or better, aerate it with an air stone for a few hours, until it has reached room temperature and lost its excess gas.
• Match the temperature. Water that is already at tank temperature has nothing left to give up. How to do a water change covers matching it.
• Spray it in rather than piping it deep: water sprayed through the air, or run onto the surface through a shower head, loses much of its excess gas on the way.
• Avoid big cold changes in winter, which release the most. Several smaller changes do the same job gently.
• Condition it, as always. Standing lets free chlorine escape along with the excess air, but it does nothing to chloramine, which many supplies now use.

For ponds, the same applies to topping up with a hose, and a pond through the year covers the summer side: shade over part of the surface, aeration that runs day and night in hot weather, and algae kept in check before it covers the water.

Bubbles that are not gas bubble disease

Several things look like bubbles and are something else entirely.

The breeds. A bubble eye carries two fluid-filled sacs under its eyes, and a crown pearlscale can carry one or two smooth bubbles on the head. Pearlscale scales themselves are domed and catch the light like beads. All are normal and permanent.

White spot. Solid, white, the size of a grain of salt, and embedded in the skin and fins, where it stays for days. It appears only on the fish, usually with flashing and clamped fins. Bubbles are clear and glinting, and outside ones are gone within hours.

Breeding tubercles. Small white bumps on the gill covers and the leading rays of the front fins of a male in spring. They are firm, they come every year, and is my goldfish breeding? shows how to tell them from disease.

A single blister. One raised, fluid-filled patch, rather than many fine bubbles, is a burn from a heater the fish rested against, or the first stage of a sore. Ulcers and sores covers what it may become.

Bubbles in the droppings. Swallowed air, usually from feeding at the surface. The constipation page covers reading droppings, and sinking food is the fix.

A fish blowing bubbles. Goldfish take air at the surface while feeding and let it out again. What is my goldfish doing? covers it, and it is normal.

Foam on the surface. A layer of foam that lingers, rather than bubbles that pop, is usually dissolved protein and waste holding the bubbles together, a sign the water is carrying more waste than it should. A water change and a smaller ration usually clear it; cloudy water covers the same load from another angle.