Goldfish vent swollen, red or protruding

The vent is the small opening on the underside of a goldfish, just in front of the anal fin, through which it passes droppings and, in a female, eggs. On a healthy fish outside the breeding season it is nearly invisible: flush with the belly and much the same colour as the scales around it. So a vent you can see at all has usually changed, and most keepers notice the day it does.

One common reason is not an illness. A female getting ready to spawn has a fuller, slightly protruding vent every spring, and she alarms more keepers than anything else on this page. The others are a sore rim from straining, inflammation of the gut, worms, a prolapse, eggs that will not come out, and things attached to or growing near the vent. They look different enough to be told apart with a torch and a few minutes of watching, and the first job is to do exactly that.

First: what is actually there?

Get a clear look before deciding anything. Wait until the fish is calm, shine a torch through the side of the tank, and look from the side and, if the fish will allow it, from slightly below. A photograph helps more than it sounds: it lets you look again without chasing the fish, and compare tomorrow with today. Photographing your goldfish covers getting one that shows the detail.

What you see	Most likely	How fast
A string of droppings the colour of the food, which falls away	Normal	Nothing to do
Long white or clear strings, day after day	Mucus from the gut	Days: see the diet
A round, full vent on a plump female in spring	Spawning condition	Watch
A small, pale, blunt cone at the vent, around spawning	The genital papilla	Watch
A pink, puffy rim on a fish passing hard droppings	Straining	Days: fix the diet
A red, swollen vent on a fish that is quiet and off its food	Gut inflammation	A vet, soon
Fine red threads that withdraw when the fish moves	Camallanus worms	A vet, this week
A glistening pink or red mass that stays out	Prolapse	A vet, today or tomorrow
A stiff, pale stick with a red sore at its base	Anchor worm	Days
A firm swelling beside the vent that grows over months	A growth	A vet, when it changes

Two questions sort most of what is left. Does it go back in? Worms withdraw, a papilla recedes after spawning, and a string of droppings falls away; a prolapse stays out. Is the fish otherwise well? A spawning female eats, swims and chases food like any other fish. An inflamed gut makes a fish quiet, and a fish that has stopped eating has a problem that no change of diet will fix.

A normal vent, and a female ready to spawn

Outside the breeding season, the vents of males and females look much the same. In condition they diverge: a male’s is small, narrow and slightly concave, a female’s rounder, fuller and slightly protruding. That difference is one of the few reliable ways of telling males from females, and it is also why a keeper who has never looked closely suddenly notices a vent one April and thinks something is wrong.

A female ready to spawn is full of eggs, and it shows. Her belly is evenly rounded, sometimes a little more on one side; her vent is full and may look pinker than the scales around it; and in the hours around spawning a small, pale, blunt tube can protrude a millimetre or two from it. That is the genital papilla, sometimes called the ovipositor, the channel the eggs leave by. Males usually give the game away by chasing her for hours, nose to her flank and vent, which the breeding page describes in full.

What makes it normal is the combination: a mature fish, the season or a recent rise in temperature, a fish that is otherwise well, and a vent that settles within days of the eggs being laid or reabsorbed. A female who is not spawned does not need help; most reabsorb their eggs over a few weeks.

What does need watching is the chasing. Several males driving one female for days can bruise and scrape her vent and belly, and a female worn down by it hides and stops eating. If her vent looks raw rather than simply full, give her a break from the males: a divider, a separate tank for a week, or a cooler, steadier temperature that damps the whole business down.

A sore vent from straining

Another common cause is at the other end of digestion. A goldfish fed mostly on dry floating food, or given more than it can pass, produces hard, bulky droppings and strains to pass them, and repeated straining leaves the rim of the vent pink and slightly swollen. Long strings trailing from the vent often go with it, and the fish is usually still bright and hungry.

The droppings tell you most. Material the colour of the food is food, and a string of it hanging for a minute before it drops is ordinary. White or clear strings, day after day, are mostly mucus, which means little is being absorbed. The constipation page sorts the colours and textures in detail, and it is the place to start if the droppings look wrong.

The fix is dietary, and it is gentle:

• A day or two without food, which a healthy goldfish takes in its stride. Fasting and rest days explains the limits.
• Soaked or sinking food afterwards, so the fish is not swallowing dry pellets that swell inside it or gulping air at the surface.
• Fibre, from blanched, shelled peas and other vegetables, a few times a week.

A vent that is sore from straining settles within days of the droppings returning to normal. If it does not, or the fish stops eating, the cause is something other than the diet.

A red, swollen vent on a fish that is unwell

When the vent is red rather than pink, the rim is swollen, and the fish is also quiet, clamped or off its food, the gut itself is more likely to be inflamed. A bacterial infection of the gut lining shows at its end: jelly-like or mucus droppings, sometimes a trace of blood, and a vent that looks angry. It is often a disease of a stressed fish in poor water, or of one fed something spoiled, and more than one fish may be affected. In cool water, a protruding vent with long, mucus-coated droppings in several fish can also be spring viraemia of carp, a notifiable disease in some countries, which is one more reason to call a vet when more than one fish is affected.

This is the case the dietary fixes do not reach. Fasting and peas help a fish that is straining; they do nothing for an infection, and a keeper who keeps trying them loses days. Test the water first, because an inflamed gut is often the second sign of a tank problem, and ammonia or nitrite above zero has to be put right whatever else is done. Then move the fish to a hospital tank and talk to a vet. The treatment for a bacterial gut infection is usually a prescription antibiotic given in food, chosen for what is there, and medications used on a guess are how resistant infections are made.

Red threads that come and go: camallanus worms

Fine red or reddish-brown threads, a few millimetres long, poking out of the vent and pulling back in when the fish is disturbed, are probably Camallanus, a parasitic roundworm, which a vet can confirm under a microscope. The adults live attached to the lining of the gut and push their tail ends out through the vent to release their young. They are easiest to see when the fish is resting, in the evening or first thing in the morning, and easy to miss if you only ever look at a fish on the move.

They are better known to keepers of livebearers and bettas than of goldfish, and no goldfish case is well documented, so a vet should confirm the diagnosis; where they do occur they arrive the same way: with a new fish that was not quarantined, and occasionally with live food collected from a pond that has fish in it. A light infection may show nothing else. A heavy one produces a fish that eats and still loses condition, with a hollow belly, and eventually a red, sore vent.

Three things matter about treating them:

• It needs a wormer, ideally chosen with a vet. The products that kill roundworms are specific, and most general parasite remedies on a shop shelf do nothing to them. Which wormers are sold, and whether they need a prescription, depends on the country.
• It is the whole tank, not the fish. If one fish carries them, the others have been exposed, and the young worms are already in the tank.
• It is usually more than one round. A single treatment rarely clears every stage, which is why a repeat a few weeks later is common. Your vet will set the schedule, and a look at every fish at rest in the evenings afterwards tells you whether it worked.

Tissue that stays out: a prolapse

A prolapse is tissue from inside pushed out through the vent: the end of the gut, or in a female the end of the oviduct. It looks like a small, glistening, pink to red bulge or tube sitting outside the vent, and what separates it from everything else on this page is that it stays out. A papilla recedes after spawning, worms withdraw when disturbed, a string of droppings falls away. A prolapse is still there hours later.

It rarely comes from nowhere. The usual story is straining: severe constipation, a difficult spawning, a female who could not release her eggs, or a gut so inflamed that the fish keeps pushing. Now and then a fish weakened by a long illness loses the muscle tone that holds the tissue in.

It is more urgent than it looks. Tissue outside the body is easily torn and exposed to the bacteria in the tank, and goldfish pick at anything that looks like food, which a small red mass does. Left alone, it swells and darkens as its blood supply fails.

While you arrange to see an aquatic vet:

1. Move the fish on its own to a bare, clean tank, away from anything that picks at it.
2. Keep the water immaculate, with small daily changes of dechlorinated, temperature-matched water, because the exposed tissue is an open wound.
3. Stop solid food for a day or two, then offer small amounts of soft, soaked food, so the fish is not straining again.
4. Do not touch it. Pushing it back, cutting it or tying anything around it does far more harm than good.

A vet can often ease a small, healthy prolapse back with the fish sedated, and sometimes place a stitch to hold the vent partly closed while it heals. They will also look for the reason it happened, which matters as much as the repair. Caught early, while the tissue is pink and clean, the outlook is reasonable. Tissue that has been out for days, has turned dark or has been bitten is far harder to save, and that is the whole argument for moving quickly.

A female who cannot release her eggs

A few females cannot reabsorb their eggs. The signs are a belly that stays swollen well past the season, a vent that is red, swollen or protruding, and sometimes a few eggs visible at it or stuck to it; she may strain, sit on the bottom or go off her food. The swollen-belly page covers telling this from constipation and from fluid, which can look much the same from above.

It needs a vet. Retained eggs can harden, become infected, and push the oviduct out as a prolapse, and the only safe ways to deal with them are the ones a vet has: sedation, a gentle and skilled attempt to express them, or surgery. Do not squeeze her. Breeders who strip eggs by hand do it to ripe females who are ready to release them, with a light touch they have practised; pressure on a fish whose eggs are stuck can rupture the organs they are packed against.

Prevention is mostly about the season. A female who spawns, or who is kept cool and steady through spring so that she never comes fully into condition, is unlikely to have the problem at all, and breeding goldfish covers both approaches.

Parasites, sores and lumps near the vent

Three more causes sit at the vent rather than coming out of it.

Anchor worm often attaches here. It is a crustacean rather than a worm, and it shows as a stiff, pale stick from a few millimetres to a centimetre long, embedded in the skin with a red, raised sore around its base. It is easy to mistake for something hanging from the vent until you look at where it is anchored. Anchor worm and fish lice covers removing it and treating the tank.

Sores and bruising come from spawning chases, from rough substrate and decor at belly height, and from the fish’s own straining. A clean pink scrape heals in good water within a week or two. A round, red, open sore that grows is an ulcer, and ulcers and sores is the page for that.

Lumps at the vent are less common. Tumours at the vent are uncommon in goldfish, but a firm swelling that keeps growing could be a tumour of the soft tissues or of the ovary, and it can push the area around the vent into a one-sided swelling that grows over months rather than days; a vet can tell which. Lumps, growths and tumours sets out what a vet can do, and caring for an old goldfish what changes with age.

Nursing a fish with a sore vent

Whatever the cause, a fish with a sore, swollen or protruding vent recovers best in the same conditions:

• Clean water above all. Test daily while it recovers. Ammonia and nitrite must read zero, and nitrate is best kept low with small, frequent changes; testing goldfish water covers which kit to trust.
• A bare, smooth tank, with nothing the fish can scrape its underside on.
• Peace. No tankmates picking at it, no males chasing a female, and as little netting as possible; handling and moving a goldfish covers doing it gently when you must.
• Soft, small meals, soaked or gel food rather than dry pellets, so the gut is not asked to pass anything bulky. Feeding a sick goldfish covers what and how much.
• A daily look and a photograph from the same angle, so you can judge whether it is really getting better.

Salt helps in some of these cases and not others, and Epsom salt is a laxative, not a treatment; aquarium salt for goldfish is worth reading before adding either.

Preventing the next one

Most of what happens at the vent can be headed off with things a keeper already controls:

• The diet. Soaked or sinking food, not too much of it, fibre from vegetables, and rest days. Straining is the root of most sore vents and most prolapses.
• Quarantine. Camallanus and anchor worm both arrive on new fish, and a few weeks apart catches them before they reach the main tank.
• Spring management. Enough room and cover for a female to escape the males, and a steady temperature if you would rather the fish did not spawn at all.
• Smooth surfaces at belly height, in the decor and in the substrate.
• A look underneath. A fish seen from below once a week is a fish whose vent you will know when it changes.