Goldfish diseases: what each one actually is

The health section of this site is organised by what you can see, because that is what you actually have when something is wrong. This page is the reference behind it: what the named diseases are, how each one works, and which ones are not diseases at all.

It is worth reading once while nothing is wrong. Read in an emergency, a list of diseases tends to produce a diagnosis by pattern-matching, which is how the wrong treatment gets chosen. Read calmly, it does something more useful — it makes clear how much these conditions have in common, and how often the same few causes lie behind them.

Finding your way in

What you are seeing	Look first at
Salt-grain white spots	Ich
Scratching with nothing visible	Flukes, or the protozoans
Grey or milky film on the flanks	Costia, Chilodonella, Trichodina
Fins fraying from the edge inward	Fin rot
Pale patch near the mouth, spreading fast	Columnaris
Open red sore, scales gone	Ulcer disease
Cotton-wool tufts on damaged tissue	Saprolegnia
Scales standing out from the body	Dropsy — a sign with several causes
Floating, sinking or rolling	Swim bladder disorder — a sign, not a diagnosis
Threads or discs attached to the fish	Anchor worm, fish lice
Waxy or cauliflower-like lumps	Lymphocystis, or a growth — carp pox is a koi and carp virus

Ich

Ichthyophthirius multifiliis, the white spot parasite, and the one condition most keepers can name.

What it is. A single-celled ciliate that burrows into the skin and gills, where it feeds and grows. Each white spot is a parasite just under the skin, wrapped in host cells.

How it works. The life cycle is the whole story. The feeding stage sits under the skin, protected, and is unreachable by anything you add to the water. It then drops off, encysts on the substrate or decor, and divides into hundreds of free-swimming young. Only that free-swimming stage can be killed. The cycle speed is temperature-dependent — a few days in warm water, considerably longer in the cool water goldfish live in — which is why treatment must continue well past the disappearance of visible spots, and why a three-day observation of a new fish can miss it entirely.

Why it appears. Ich cannot survive without living fish, so it arrives on a new fish, or on wet equipment and plants from a tank that holds fish, which is the argument for quarantine. A temperature crash or a stressful move is the classic point at which it breaks out.

The symptom page is white spots.

Flukes

Monogenean flatworms — Gyrodactylus on the skin, Dactylogyrus on the gills — and a common answer to unexplained irritation.

What they are. Microscopic worms that attach with hooks and feed on skin or gill tissue. Invisible without magnification, which is precisely what makes them so often missed.

How they work. The two genera differ in a way that matters for treatment: the skin fluke bears live young, while the gill fluke lays eggs that are resistant and hatch later, which is why the Merck Veterinary Manual describes multiple treatments at weekly intervals for egg-laying monogeneans. It is also one reason a fluke treatment that appears to work can fail a fortnight later: it cleared the adults and met the next generation.

Why they appear. Flukes are obligate parasites, so they arrive on a new fish or with plants and wet equipment from another tank, and low numbers on a fish can cause little. They multiply when fish are crowded, stressed or in poor water, and a heavy burden can be lethal, particularly on the gills. Suspect them when a fish flashes persistently with clean water and nothing visible, especially with fast breathing.

Costia, Chilodonella and Trichodina

Three small protozoan parasites grouped together because they present alike and cannot be separated by eye.

Costia (Ichthyobodo) is a tiny flagellate that causes intense irritation and a heavy grey slime response, and it can do serious damage. It does well in cool water, which suits a goldfish tank.

Chilodonella is a ciliate producing a similar grey-white film, also favouring cooler water, and capable of killing quickly in a heavy infestation.

Trichodina is a disc-shaped ciliate, often described as looking like a flying saucer under a microscope. It is frequently more a symptom than a cause: a heavy Trichodina count usually indicates a tank with high organic loading, and improving the water often resolves it.

All three share a picture: flashing, clamped fins, a dulled or milky appearance from excess slime, and fish that are clearly uncomfortable with nothing visible on them. All three need a scrape to identify. Grey film and excess slime works that shared picture from the other end, starting from what the keeper can see and separating it from fungus, ich and columnaris.

Fin rot

What it is. Not a single organism. Fin rot is opportunistic bacterial colonisation of fin tissue — usually Aeromonas, Pseudomonas or Flavobacterium species, all of which are common in aquaria — on a fin that is already compromised.

How it works. The bacteria attack from the fin edge, the point furthest from the fish’s blood supply and immune defences, and work inward. The margin becomes ragged, frayed or melted-looking, often with a white or reddish rim, and it advances visibly over days.

Why it appears. Often water quality. The organisms are common in aquaria and take hold when the fish’s resistance drops or the fin is damaged. This is why mild fin rot often resolves on clean water alone, and why medicating without fixing the water often fails.

The symptom page is torn fins.

Columnaris

Flavobacterium columnare, historically called mouth fungus, and a dangerous one.

What it is. A bacterium, despite the name and the fungal appearance. It produces pale, flat, spreading lesions, classically around the mouth, on the gills, or as a whitish “saddle” across the back just in front of the dorsal.

How it works. Fast. An acute columnaris outbreak can kill within a day or two, particularly in warmer water, and the gill form can kill with very little external sign at all. It is contagious between fish.

Why it matters here. It is frequently mistaken for fungus and treated with an antifungal, which does nothing. Speed and site are useful clues without a laboratory: fungus usually sits on damaged tissue, while columnaris can spread across intact skin and move. A fish with a rapidly spreading pale lesion is a reason to contact a vet today rather than to work through a shelf.

Ulcer disease

What it is. Deep bacterial infection of the skin and underlying tissue, most often Aeromonas species, producing the crater-like sores described on the ulcers page.

How it works. Bacteria enter through a break — an injury, a parasite wound, chemical damage — and erode outward from that point. Left in poor water it progresses to septicaemia, at which stage the fish shows red streaks, lethargy and loss of appetite, and the problem is systemic rather than superficial.

Why it matters. It is the condition where the gap between shop treatment and veterinary treatment is widest. A systemic bacterial infection needs an antibiotic chosen against the organism, ideally after a swab, which in many countries requires a prescription — how to find an aquatic vet is the practical side of that.

Dropsy is not a disease

This distinction is worth stating plainly, because treating dropsy as a diagnosis is a common and costly error.

What it is. Dropsy is a sign: fluid accumulating in the body cavity and tissues until the scales lift away from the swollen body, producing the pinecone appearance. It describes a state, not a cause.

What causes it. Several things. A goldfish kidney parasite, Sphaerospora auratus, causes what the Merck Veterinary Manual calls renal dropsy of goldfish, for which no practical treatment is available. Bacterial infection and viral disease, including the goldfish herpesvirus CyHV-2, can produce the same swelling, as can failure of the kidneys, liver or heart after a tumour or chronic organ damage. The osmoregulation section of the anatomy page explains why a freshwater fish that loses this control accumulates water rather than losing it.

What it means practically. There is no single “dropsy treatment”, because dropsy is a sign with several causes, and the cause decides the outlook. Some have no practical treatment; others, such as a bacterial infection caught early, may be reversible; and by the time scales lift the problem is often well advanced. It is a veterinary case, not a shop one.

The symptom page is swollen abdomen.

Swim bladder disorder is not a disease either

The same problem with the same consequence.

“Swim bladder disorder” describes a fish that cannot hold its trim. It is common in round-bodied fancies, and diet and swallowed air are suspected contributors: air taken in at the surface, a gut distended with food pressing on the bladder, or constipation slowing everything down. Infection, injury and anatomy, including a deformed bladder, also occur, and occasionally the cause is neurological. No study gives the proportions of each.

Keepers report that a day without food, a switch to sinking food and some fibre help some fancies; there is no controlled evidence, and because infection is among the causes, a fish that does not improve needs a vet. Antibiotics are a vet’s decision, not a reflex for every floating fancy. The floating upside down page works through the distinction.

Fungus

What it is. Saprolegnia and related water moulds — not true fungi in the strict sense, but treated as such practically. Their spores are common in aquaria.

How it works. It is usually a secondary invader, colonising tissue that is already dead or damaged rather than healthy skin. The classic appearance is discrete cotton-wool tufts on a wound, a rot lesion, or on dead eggs in a spawning tank.

Why it matters. Fungus is often a marker rather than a primary problem: its presence says there is damaged tissue, and often that the water has been poor enough to let moulds establish. Treating the fungus while leaving both causes in place achieves very little.

Anchor worm and fish lice

The two parasites large enough to identify with confidence.

Anchor worm (Lernaea) is a parasitic copepod — a crustacean, despite the name — whose adult female embeds her head in the fish’s flesh and leaves a thread-like body protruding, with two egg sacs at the free end. The embedded head leaves an inflamed crater that readily becomes an ulcer, which is often the more serious problem. Removal is a job to be done carefully, and the site of removal needs watching.

Fish lice (Argulus) are flat, translucent, disc-shaped crustaceans a few millimetres across that move about the body, pierce the skin to feed and leave small wounds. They are visible, and they swim, so they can move between fish.

Both are classic pond and new-stock arrivals rather than things that appear spontaneously in an established indoor tank, and the anchor worm and fish lice page covers identifying, removing and treating them.

Velvet

Piscinoodinium, a parasitic dinoflagellate, producing a fine gold or rust-coloured dusting over the body that is easiest to see with a torch held at an angle.

It is more a tropical fish problem than a goldfish one, but it occurs. The distinguishing feature against ich is particle size: ich spots are discrete grains you can count, while velvet is a fine dust that reads as a sheen on the skin rather than as individual points.

Carp pox and lymphocystis

Three viral conditions, and they mean very different things for a goldfish.

Carp pox is caused by a cyprinid herpesvirus (CyHV-1) and produces smooth, raised, waxy, candle-wax-like growths on koi and common carp, tending to come and go with the water temperature. The Merck Veterinary Manual lists common carp and koi as the only susceptible species, so it is not known to infect goldfish, and a lump on a goldfish cannot be named by eye — lumps, growths and tumours sets out how to work through the possibilities.

Goldfish herpesvirus (CyHV-2) is the one that makes goldfish ill, and it is serious: pale gills, swelling of the abdomen and heavy losses, sometimes within days, with no treatment.

Lymphocystis produces cauliflower-like clusters of enlarged cells, usually on fins. It is viral, benign and self-limiting over weeks or months.

Texture and pace help sort lumps: waxy or granular rather than fluffy, and stable or fading rather than steadily growing, both point away from fungus and tumours. None of it names a virus by eye.

Internal parasites

The category that is hardest to recognise, because everything happens out of sight.

Intestinal worms — tapeworms, nematodes such as Camallanus, and others — live in the gut and take a share of what the fish eats. The classic picture is a fish that eats normally and steadily loses condition, thin along the back while still behaving well, often passing long, pale, stringy faeces. Camallanus occasionally shows itself directly as fine red threads protruding from the vent, which is one of the few internal parasites you can actually diagnose by looking.

Flagellates in the gut, including Hexamita, cause wasting and white stringy faeces in a similar pattern. Hexamita is far better known as a cichlid problem than a goldfish one, but it exists here.

Two points make this group distinctive. First, they arrive with live food, plants and new fish rather than appearing spontaneously, which is another entry on the quarantine argument and a reason to source live food carefully. Second, treatment has to reach the inside of the fish, which usually means medicated food rather than anything added to the water, and that needs a fish that is still eating; a vet can also use baths or injections for one that has stopped. That is one more reason a declining appetite is worth acting on while it is only a decline.

Bacterial gill disease

A condition that kills without ever looking dramatic, and therefore worth naming.

The gills are among the most exposed tissues a goldfish has, described in detail on the anatomy page, and they can become bacterially infected and inflamed in their own right — often Flavobacterium species, often on gills already irritated by ammonia, nitrite or crowding. The lamellae swell and fuse, the surface area for gas exchange collapses, and the fish suffocates slowly in water with plenty of oxygen in it.

What makes it dangerous is the absence of external signs. There is no lesion to see. The fish breathes fast, holds near the surface or the filter outflow, becomes lethargic and goes off its food — a picture identical to a dozen other things, and identical in particular to simple low oxygen. The practical rule is that rapid breathing which does not improve when aeration is increased and the water tests clean is a gill problem until proven otherwise, and gill problems deteriorate faster than skin ones.

Tumours and growths

Goldfish are long-lived animals, and long-lived animals get tumours. In an older fish a slowly enlarging lump is more likely to be a growth than an infection.

They appear anywhere — on the body wall, at the base of a fin, inside the mouth, occasionally on the gill cover — and range from benign fatty masses and fibromas to genuine malignancies. The distinguishing features against the infectious conditions above are pace and surface: a tumour grows over months rather than days, and is usually covered by intact skin rather than being an open lesion or a surface deposit.

Most need nothing. A benign mass that is not obstructing feeding, swimming or vision is better left alone in a fish that is otherwise well. Where one does interfere, surgical removal under anaesthetic is available from an aquatic vet. What is never useful is treating a growth with antibacterial or antiparasitic remedies.

One special case: an ulcerated growth, where the surface has broken down, is both a tumour and an open wound, and the wound half needs the ulcer response while the vet deals with the rest.

How disease actually arrives

Most outbreaks have a route in, and there are only a handful.

A new fish. A common route, and the one quarantine is for. Shop systems are often shared water, and a fish that looks well can be carrying several of the conditions on this page. At least four weeks of quarantine addresses this route.

Plants. Snails, snail eggs, fluke eggs and protozoan cysts all travel on plants from a tank containing fish. The plants guide covers the handling.

Live food. Wild-caught live food comes from water containing fish, and therefore from water containing fish parasites. Cultured food from a clean source does not.

Shared equipment. A net or siphon moved between tanks carries water, and water carries everything in it. Separate equipment, or dry it completely between uses — most of what you are avoiding does not survive drying.

Pond water and pond fish. Ponds carry a wider range of parasites than indoor tanks, and anything moving from pond to tank should be treated as new stock.

Nothing new. The remaining category: an opportunist already in the tank, such as Aeromonas, Pseudomonas, a water mould or Trichodina, takes hold when the fish’s resistance falls. This is the route that quarantine cannot close and water quality can.

A spine that is slowly curving on a fish that is losing weight while still eating can point to a slow infection with an organism that was already in the water — a bent or curved spine covers how to read that.

Principles of treating any of them

Where treatment is genuinely indicated, the principles are consistent across everything above — and using medications safely works through each of them in full, including how to arrive at a real water volume and what a course does to the filter:

• Treat in a hospital tank where possible, not the display. It protects the filter, uses far less medication, and lets you observe the patient properly. Setting up a hospital tank covers the tank and how to run one that has no cycle.
• Remove activated carbon from the filter first, or it will strip the medication you just added.
• Increase aeration. Many treatments reduce dissolved oxygen. Goldfish tolerate low oxygen better than most aquarium fish, but a sick fish in a warm, medicated tank can still exhaust the margin.
• Dose against real volume, not the number on the tank’s box, which ignores substrate, decor and the gap below the rim.
• Finish the course and match it to the life cycle. Stopping when the visible signs disappear is how ich returns — the encysted stage was never touched.
• Do not combine treatments without knowing they are compatible. Several common combinations are dangerous.
• Fix the cause. Many organisms on this page are opportunists, let in by stress that stays, and the rest take hold more easily on a stressed fish. A treatment that clears the symptom and leaves the water unchanged is a postponement.

Where to go next

Start from what you can see: the health section covers each visible sign, its likely causes and what to check first. If nothing appears wrong but you are not sure, what a healthy goldfish looks like is the reference for normal — including the several alarming-looking things on this list’s borders that turn out to be nothing at all.