A goldfish losing weight or getting thin

A goldfish that is getting thinner has one of two problems. Either it is not getting enough food, or it is not keeping what it eats. They look much the same from the front of the tank and they need opposite answers: the first is solved by feeding it better, and the second is made worse by feeding it more.

Most thin goldfish have the first problem. They come forward at every feeding, get beaten to it, and fade slowly in a tank that looks well fed. The second is less common and more serious: a fish that eats its share and loses weight anyway is feeding something else, most often a parasite. Telling the two apart takes one feeding, watched properly.

First: is it really thinner?

Weight loss is noticed late, because it is looked for from the wrong side.

Look from directly above. A healthy goldfish seen from overhead is evenly wide behind the gill covers and tapers smoothly to the tail. A fish losing condition narrows there first, so it looks pinched behind the head, and in time the back develops a thin ridge, the knife edge keepers describe. Seen from the side, the body keeps its depth for a long time, which is why a fish can lose a great deal before anyone notices.

Look at the belly and the head. A belly that looks hollow behind the pelvic fins, and a head that looks too large for the body, are both signs of a fish that has lost condition.

Compare it with itself and with the others. A photograph taken from above every week or two, against the same background, shows a change that is invisible day to day. Photographing your goldfish covers taking one that is worth comparing. A fish of similar age and breed in the same tank is the other good reference.

Some changes look like weight loss and are not:

• A female that has just spawned is suddenly slimmer because the eggs have gone. It happens overnight, in spring, usually after days of chasing, and is my goldfish breeding? describes it.
• A pond fish just out of winter has been eating little or nothing for months and fills out again as the water warms; feeding through the seasons covers the pond year.
• A young fish growing fast often lengthens before it fills out.
• A single-tail is slim by build. Comets, commons and shubunkins are streamlined fish, and one kept beside an oranda is not thin, just different.

Then: is it eating, and is it getting the food?

Watch a whole feeding from across the room, where the fish cannot see you and start begging, and follow the thin fish rather than the food.

What you see	Most likely	Where to go next
It does not come to the food, or comes and turns away	Not eating at all	Loss of appetite
It comes forward and is beaten to almost every mouthful	Losing out at feeding time	Feed it better, below
It tries to eat, misses or spits food out repeatedly	A mouth or eye problem	Mouth problems
It eats its share and thins anyway	Internal parasites	A vet and a faecal sample
Thin along the back, still round in the belly	Parasites, or organ disease in an older fish	A vet
Several fish thinning over months, with bent spines or sores	Mycobacteriosis	A vet, and care with your hands
An old fish, slowly, over a year or more	Age	Softer food and patience

A fish that has stopped eating altogether has a different problem from one that is eating and thinning, and the page on a goldfish not eating is where to start. The rest of this page is about the fish that still wants its food.

Losing out at feeding time

A goldfish tank is a competition every time food goes in, and the fast fish win it. The pattern is common and it has nothing to do with health:

• Fancies against single-tails. Commons and comets reach food first every time, and a round-bodied fancy kept with them can thin for months while looking hungry. It is one of the reasons the two types are best kept apart, and how many goldfish can I keep covers the rest.
• Poor eyesight. Telescopes, moors, celestials and bubble eyes find food slowly, and a fish with cloudy eyes finds it slower still.
• An old fish among young ones, which is slower and less pushy than it was.
• A fish that is chased. One persistent bully can keep another away from the food for weeks; the page on chasing and bullying covers telling harassment from spawning.

The fixes are simple, and none of them involves feeding the tank more:

1. Feed in two places at opposite ends of the tank, so the fast fish cannot cover both.
2. Use sinking food, which reaches the bottom where slow fish feed instead of vanishing at the surface.
3. Feed the fast fish first, at one end, then the slow one at the other.
4. Put food in front of it. A baster or a long pipette can place a soaked pellet or a piece of gel food right at a slow or poorly sighted fish’s mouth.
5. Feed it apart if nothing else works: a separate tub of tank water for ten minutes at mealtimes, or a spell in a hospital tank while it recovers.

Not enough food, or not the right food

Sometimes every fish is getting the food and the ration is simply wrong.

Too little. Warnings about overfeeding are everywhere, and they are right, but a keeper can overcorrect. A growing juvenile needs considerably more food for its size than an adult, and a fish in warm summer water needs more than the same fish in a cool room. How often to feed goldfish sets out what a normal ration looks like, and the feeding calculator below works it out for your fish and temperature.

Too poor. Flakes and pellets lose their vitamins within months of opening, and a diet of one stale food can keep a fish eating without keeping it in condition. A fresh tub, a varied diet and some fresh foods make the difference; goldfish nutrition covers what a goldfish actually needs.

Eating well and still getting thinner

This is the picture that matters, and the one where feeding more is the wrong answer.

A goldfish that eats a normal amount, gets its share and still narrows along the back over two weeks or more is very likely carrying an internal parasite: tapeworms, roundworms or single-celled gut flagellates, taking the food after the fish has eaten it. The droppings often give it away. Pale, white or clear strings day after day are mostly mucus, not digested food, and the constipation page sets out how to read them. Fine red threads at the vent are camallanus worms, which the vent page covers.

The answer is a diagnosis, not a guess. An aquatic vet can examine a fresh dropping under a microscope and identify what is there, and the treatment depends entirely on what it is: the products that clear a tapeworm do nothing to a flagellate, and neither touches a roundworm. Finding an aquatic vet covers how to find one and what to bring. The treatment is usually a drug given in food, while the fish still has an appetite, which is one more reason to act before it stops eating; feeding a sick goldfish covers medicated food in more detail.

Parasites rarely stop at one fish. Treat the tank as exposed, and look for where they came from: most arrive with a new fish that was not quarantined, and some with live food collected from a pond that holds fish.

Slowly, over months, and more than one fish

One cause is worth knowing about because it changes what happens next, not because it is likely.

Mycobacteriosis, sometimes called fish TB, is a slow bacterial infection that wastes a fish over months while it is still eating. It rarely arrives alone: a spine that starts to curve, ulcers that will not heal, fading colour or a bulging eye often come with it, and a second fish may start the same slow decline some time after the first. The bent spine page sets out when to suspect it, and goldfish diseases explained covers what it is.

It is not treatable in any practical way, and only a vet can confirm it, often only after a fish has died. That makes a post-mortem on a fish lost to slow wasting more useful than it sounds: it is how the fish still alive are protected. Why did my goldfish die? covers arranging one.

It also matters for the keeper. The bacteria involved can infect people through cuts and grazes, causing a slow skin infection on the hand or arm. The precautions are the ones worth taking with any tank: keep broken skin out of the water or wear gloves, wash your hands after working in the tank, and tell your doctor you keep fish if a sore on your hand will not heal.

Other causes, and a fish that is simply old

Chronic stress. A fish living with high nitrate, low oxygen, too many tankmates or constant chasing burns condition and usually eats less as well. The thinning is slow and rarely confined to one fish. Testing goldfish water covers what to measure and what the numbers mean.

Trouble eating. A sore or damaged mouth, or poor sight, keeps a fish trying and failing. It looks like competition, but the fish loses out even when it is fed alone, which is the test.

Organ disease and internal growths. More common in older fish, these can waste the body while the belly swells, so the back is thin above a round abdomen. Lumps, growths and tumours sets out what a vet can do.

Age. A goldfish in its second decade loses condition gradually, eats less and rests more, over months or years rather than weeks. Softer food, smaller pieces, more frequent small meals and a little protection at feeding time help more than anything else, and caring for an old goldfish covers the last years in full. What is not age is a decline over days, or one that arrives alongside anything else.

Putting condition back on

Whatever the cause, a thin fish recovers slowly, and the recovery is a phase of its own.

• Small meals, more often. Two or three small meals a day beat one large one, and the gut of a fish that has been short of food handles them better.
• Good food. This is when quality earns its keep: a varied diet with a good staple, some fresh foods, and gel food, which is soft, easy to eat and easy to enrich.
• Feed it where it can eat. If it has been losing out, a spell in a hospital tank lets it eat without competition, and lets you see exactly how much it takes.
• Keep the water clean. More food means more waste, and a fish putting condition back on does it better in water that is not working against it.
• Measure it. The same photograph from above every week shows whether it is working. Condition returns over weeks, and a young fish that was thin through its growing months may never fully catch up.
• Then stop. Once it looks like its tankmates again, it needs an ordinary ration, not the generous one that got it there.

A fish that eats well, gets its share and still does not gain over two or three weeks has told you the problem is not the food.

Preventing it

• Watch one feeding a week with the thin fish in mind: who reaches the food, and who does not.
• Keep fancies and single-tails apart, and give slow or poorly sighted fish a way to eat.
• Look from above once a month, and keep a photograph.
• Quarantine every new fish, which is how most internal parasites are kept out.
• Feed fresh, varied food, and replace a tub that has been open for many months.
• Test the water, because chronic poor water thins fish slowly enough that nobody notices until several have gone.