Goldfish with clamped fins

A goldfish in good spirits swims with everything spread: dorsal fin up like a sail, tail fanned wide, pectorals sculling. A goldfish that is uncomfortable folds all of it away. The tail narrows to a point, the dorsal lies flat, and the fish looks suddenly smaller and meaner than it did yesterday. Clamped fins are not a disease. They are the goldfish version of hunching your shoulders against the cold, a posture that says something is wrong without saying what. Reading the rest of the fish, and testing the water, usually tells you the rest.

Telling the causes apart

Start by counting. If every fish in the tank is clamped, the cause is almost certainly shared, which means the water or the temperature. Ammonia and nitrite in a new or overstocked tank, nitrate that has climbed for months without a water change, a pH that has slipped, a heater that failed overnight, or a water change done several degrees cooler than the tank all produce a tank full of folded fins within hours. There is usually a story, and you usually know it.

If only one fish is clamped, look at that fish and its neighbours. A new arrival clamped for its first day or two is settling in. A fish that is being chased, nudged or squeezed out at feeding time is being bullied, and the fix is space or a rehome, not medicine. A fish that flicks against the substrate, carries a grey sheen of extra slime, breathes hard and rests between bouts of restlessness is probably carrying a parasite. If it has white spots, you can name the parasite. If it has none, the parasite is microscopic and a vet’s skin scrape is the way to identify it.

A fish that is clamped in clean, warm, calm water, with no bully and no flicking, but is also pale, off its food and losing condition, may be in the early stage of a bacterial infection. Watch it closely for red streaks, bloating or sores over the next few days.

What to do first

Test ammonia, nitrite, nitrate and pH, and check the temperature against what it was a few days ago. Anything out of range gets fixed first: a 30–50% water change (at least half if ammonia reads 2 mg/L or more, unless the pH is below about 6, when smaller daily changes are safer) with conditioned, temperature-matched water for ammonia, nitrite or high nitrate, repeated daily until the readings are right; a gradual correction for temperature, no faster than about 1 °C an hour and slower for a sick fish. Our guide to testing goldfish water explains what each reading means and how often to check.

If the water is fine, watch the tank for ten minutes at feeding time. Chasing, crowding and one fish hanging back tell you about bullying. Flicking and rubbing tell you about parasites. A fish that unfolds its fins and eats normally, then clamps again afterward, is far less worrying than one that ignores food altogether.

Seeing the slime film

One of the most useful signs on this page and the hardest to see, so it is worth describing properly.

A fish whose skin is irritated, by a parasite or by something in the water such as chlorine, produces extra mucus, which shows as a dull, grey or faintly bluish sheen that breaks up the normal metallic shine. It is often patchy rather than even, and usually clearest along the back and just behind the head.

How to see it: turn the room lights down and rake a torch along the flank from the side, so the beam travels across the fish rather than at it. Shone head-on, the light reflects and hides exactly the surface texture you want. Compare against a fish in the same tank that is not clamped, if you have one.

It matters because it narrows the field. A clamped fish with clean water and a normal sheen is stressed by something environmental or social. A clamped fish with a slime film has irritated skin, which means a parasite or something in the water such as chlorine, so test the water first.

The parasites you cannot see

Ich announces itself. The others do not, and they are the reason a clamped fish in clean water is worth taking seriously.

Costia, trichodina, chilodonella and skin flukes are all microscopic, all irritate the skin and gills, and all produce roughly the same picture: clamping, flashing and rubbing, extra slime, faster breathing, and a fish that alternates between restlessness and sitting on the bottom.

Three things are worth understanding about them.

Some are usually already present. Trichodina and Chilodonella can persist at low levels in ordinary tanks and bloom on a fish whose defences are down — which is to say, after the water has been poor, the temperature has swung, or the fish has been moved. Flukes are different: they have to arrive on a fish, a plant or some water, and then build up quickly. That is why the water test comes first even when a parasite is obviously involved: the parasite is often the consequence rather than the cause, and where it is a fluke it is the reason for quarantine.

They cannot be told apart by looking. The behaviour they produce is nearly identical, and the treatments are not interchangeable. A vet identifies them from a skin or gill scrape under a microscope in a few minutes, which is a short appointment and the difference between treating and guessing. Finding an aquatic vet covers locating one.

Treating blind usually fails, twice. The first failure is that the wrong compound does nothing. The second is that a course of medication is itself a stressor, often reduces dissolved oxygen, and can knock back the filter — so a fish that was clamped and stable becomes a fish that is clamped and in a tank with an ammonia reading. Using medications safely covers the part people skip.

And they arrive with new fish, more often than anything else. This is the whole argument for quarantine, and a clamped fish two weeks after a new arrival is close to a confession.

Confirming a tank mate is the cause

Bullying is a commonly guessed-at explanation and rarely confirmed, so it is worth watching properly rather than assuming — chasing and bullying covers what it usually turns out to be instead.

Watch at the two moments it shows. Feeding time, and the first few minutes after the lights come on. Competition is at its most visible then, and a fish that looks fine at three in the afternoon can be having a bad life at eight in the morning.

What you are looking for:

• One fish consistently reaching food first and driving another off it — the second half matters, because fast eating is not aggression.
• Repeated nudging at the flank or vent, which in spring is usually spawning behaviour rather than aggression — is my goldfish breeding describes the difference, and it is vigorous enough to alarm anyone.
• One fish keeping to a corner and moving away when approached.
• Damage on that fish and not the others: fresh splits in the trailing fin edges, missing scales along one flank, or damage that keeps reappearing after it heals. This is the clearest evidence there is.

What to do about it:

• More space. Most goldfish “aggression” is crowding, and the stocking figures are the first thing to check.
• Cover and sight lines. A plant, a smooth ornament or a piece of hardscape that breaks the line of sight lets a harassed fish get out of view, which is worth more than litres.
• Feed in two places, at opposite ends, so a slow fish is not competing for a single spot.
• Separate them if it continues. A spawning male will pursue a female for days, and a few days apart is reasonable; a mismatch between a fast single-tail and a slow fancy is permanent and needs rehoming one of them.

Settling in, and when it stops being that

A new fish clamped on its first day is normal. Knowing when that stops being the explanation saves both unnecessary worry and dangerous patience.

The ordinary sequence:

• Day 1. Sitting low, fins folded, uninterested in food. Expected, and the right response is dim light and no attention.
• Day 2. Beginning to move around, investigating, possibly eating.
• Day 3. Eating properly, fins carried normally most of the time.

The pattern rather than the clock is what matters. A fish that is progressively more active and interested each day is settling. A fish that is the same on day four as on day one, or that eats and then immediately clamps again, is not adjusting to anything — it is telling you about the water, or about something it brought with it.

Two things make settling harder than it needs to be: a tank that is still cycling, which is common when the fish and the tank arrived the same week and is covered on the cycling page, and being watched. A new fish in a busy room with faces at the glass has no way to decide it is safe.

Temperature swings

Underrated as a cause, because nothing looks wrong afterwards.

Goldfish tolerate a wide range of temperatures and tolerate changes in temperature considerably less well. A swing of more than about two or three degrees in a short period is a real stressor, and the fish shows it by clamping for a day or two and then recovering — which means the cause has usually disappeared by the time anyone investigates.

The common sources:

• A water change done several degrees cooler than the tank. A frequent version, and entirely avoidable by matching by hand before the water goes in.
• A heater that failed, or one that is undersized for a cold room.
• A hot afternoon on a tank in direct sun, or near a radiator.
• A window left open in winter, next to a small tank.

The temperature guide covers where the water should sit, and heaters and cooling covers holding it there. The principle throughout is that large, fast swings do the harm — no study has ranked a steady 16 C against a 20 C that visits 24 on sunny afternoons, but it is sudden changes that outrun a fish’s ability to adjust.

When it is something else

A fish that stays clamped after the water and the temperature have been right for several days has something the tests do not cover, and the parasites above are the usual answer. Two situations should not wait for the rest of the week, though. A clamped fish that is also gasping at the surface in well-aerated water has gills under strain. And a clamped fish that is pale, off its food and losing condition in clean water may be in the early stage of an internal infection, which shows in the fins before it shows anywhere more obvious.

Preventing it

A goldfish that never has to cope with ammonia, nitrite, swings in temperature or a bully will very rarely clamp. That means a cycled filter before the fish arrives, a tank sized for the number of fish in it, a stable temperature away from windows and radiators, and quarantine for every new fish.

Two of those deserve more than a mention, because clamping is what they produce first. A water change is a common avoidable cause: new water noticeably off the tank’s temperature, or a conditioner mixed wrongly, can leave a fish clamped. Mix to temperature by hand before it goes in and dose the conditioner as the label says, for the water you add or straight into the tank, and the change stops being an event the fish has to recover from. The second is flukes, which arrive on new fish and produce clamping long before anything else is visible, which is the entire argument for at least four weeks of quarantine rather than a token few days.

Clamping is a signal, so the real prevention is noticing it. Unlike most of what this site covers, this one is not a condition to avoid but a warning to catch, and it is only useful if someone is looking. A minute a day at feeding time is enough: fins open, fish moving, all present. A tail that clamps on Tuesday and is still clamped on Wednesday has told you something is wrong while every parameter still reads fine and days before the fish looks ill — which is the cheapest warning you will ever get, and worth nothing if nobody takes it.