Koi or goldfish? How to tell them apart, the fish in between, and one pond for both

A small koi and a small goldfish can be hard to tell apart. Both are orange, or red and white, or mottled; both are sold as “pond fish” out of the same tanks; and at five centimetres both look like the fish most people picture when they think of a pond. The difference only shows over the years that follow, when one of them stays at the size of a garden fish and the other passes half a metre and several kilograms.

That is why it matters which one you have. A goldfish has a place in a properly sized tank or a pond. A koi, whatever size it is today, needs a large pond and nothing less. The two are also different animals, in different genera, domesticated in different countries from different wild fish, and they can still cross, which is where the fish that fits neither description comes from.

The one-second answer: look at the mouth

A koi has barbels, and a goldfish has none. Barbels are the fleshy whiskers carp use to feel and taste for food in the mud. A koi carries two pairs on its upper lip: a longer pair at the corners of the mouth and a shorter pair just in front of them. A goldfish’s lips are bare. It is the single most reliable difference between the two, and it is the one worth checking before anything else; goldfish senses covers why goldfish taste with their lips and skin rather than with barbels.

On a young fish the barbels are short, and the second pair can be tiny. Look from the side in good light, with the fish level with your eye, or wait until it comes to the surface to feed, when the mouth is pointed at you. A phone photograph, enlarged, will usually show them better than your eye does. There is no need to catch the fish.

The mouth itself helps too. A koi’s mouth sits low and opens downward, built for rooting in the bottom; a goldfish’s sits at the front of the head and opens forward. Once the barbels have told you which you have, the difference in the mouth becomes easy to see on the next fish.

Everything else, and why it does not settle it

Every other difference between koi and goldfish overlaps somewhere, which is why the barbels come first. The rest are useful for confirming what the mouth has told you.

Feature	Koi	Goldfish	Settles it?
Barbels	Two pairs on the upper lip	None	Yes
Mouth	Low, opening downward	At the front, opening forward	Nearly
Scales along the side	Smaller, about 32–38 in the lateral line	Larger, about 25–31	On a calm fish, yes
Body	Long, thick through the back	Comets long but slimmer; fancies round	No
Colour	Red, white, black, yellow, metallic	Largely the same range	No
Long fins	Butterfly koi	Comets, shubunkins, many fancies	No
Adult size	60–90 cm, several kilograms	15–30 cm for most, rarely more	Only in time

The table’s last row is the cruel one. By the time size tells the two apart, the fish has already outgrown the tank it was bought for.

Colour is the difference people look at first, and it is the least useful. Koi and goldfish have been bred for the same colours for centuries. A young kohaku koi, white with red patches, and a red-and-white comet can look almost identical in a shop tank. A calico shubunkin and a young sanke koi, red, white and black, are not far apart either.

Long fins mislead in the same way. Butterfly koi, a long-finned line developed in Japan in the early 1980s from koi crossed with long-finned Asian carp, and named in the United States, have flowing fins that look very like a comet’s or a long-finned wakin’s. The barbels are still there, at the corners of the mouth.

Body shape separates a koi from a fancy goldfish at a glance, but not from the single-tails, which are the goldfish it gets confused with. A common goldfish is long and streamlined, and a young koi is only a little thicker through the back.

From above: telling them apart in a pond

In a pond you rarely see a fish from the side. You see backs, from above, through a surface that moves, which is how koi are judged and not how goldfish usually are. A few things still work from there.

Feeding time is the best chance. A fish taking food at the surface points its mouth up at you, and the barbels at the corners of a koi’s mouth show clearly against the water. A handful of floating food and a minute of patience answer the question for most of the pond.

The head. Seen from above, a koi’s head is broad and blunt, and the body behind it keeps that width a long way back before it narrows. A single-tailed goldfish’s head is narrower, and the body tapers sooner.

The tail. A comet’s tail is deeply forked, with long pointed lobes, and it is often as long as a third of the fish. A koi’s tail is shorter and more evenly cut, except in butterfly koi, where everything is long.

The way they move. Koi cruise slowly, in a steady line, and spend a good deal of time rooting at the bottom and the pond walls. Comets and shubunkins move in quicker bursts and change direction more. It is a tendency rather than a rule, but in a mixed pond it is often what you notice first.

The size range. A pond where some fish are twice the length of the others by their second or third summer usually holds both. Goldfish grow quickly in a pond, and koi grow much faster.

None of these is as good as the barbels, but together they will usually tell you which fish to look at more closely next time they come up to feed.

Names that mislead

A good deal of the confusion is in the names rather than the fish.

“Koi” goldfish. Koi ryukin, koi oranda and koi ranchu are goldfish. The word describes a colour pattern, patches of red, white and often black laid over the body the way a koi’s are. These fish have no barbels, and they are exactly the breeds their second names say: a koi ryukin is a ryukin, with a ryukin’s round body and a ryukin’s needs.

Tancho. In koi, tancho is a white fish with a single round red patch on the head, named after the red-crowned crane, and often compared to the Japanese flag. Goldfish borrowed the word for white fish with a red cap, most often orandas. Two very different fish can carry the same name.

Sarasa, calico, kohaku. Sarasa is the goldfish word for red and white; kohaku is the koi word for much the same colouring. Calico is used in both hobbies. Goldfish genetics explains what those colours are made of in a goldfish, and why a pattern on a juvenile is not the pattern it will keep.

Ghost koi. A cross between a metallic koi and a wild-type carp, dark with a metallic sheen. It is a carp, not a goldfish, and it grows like one.

The fish in between: koi-goldfish hybrids

Koi and goldfish are both carp, in the same family and the same subfamily. They spawn at the same time of year, when the water warms in spring, in the same way, with males driving females through the plants until the eggs are scattered. In a pond that holds both, eggs from one can be fertilised by the other, and hybrids can turn up in mixed ponds that are left to breed.

A hybrid looks like a compromise between its parents:

• Barbels absent, reduced to one small pair, or two pairs so short they are easy to miss. A “goldfish with whiskers” is the classic hybrid.
• Body longer and thicker than a goldfish’s, slimmer than a koi’s.
• Scales smaller than a goldfish’s, and more of them along the side.
• Colour often a bronze or olive like a wild fish, or a patchy version of the parents’ colours.
• Size larger and faster-growing than the goldfish parent.

Hybrids are healthy fish, and nothing needs doing about one. They are often sterile or only partly fertile (most first-generation males are sterile), but fertile hybrids and backcrosses occur in culture and in the wild. The main question is how much room it needs as it grows; plan for something closer to a koi than a goldfish.

If hybrids are not wanted, the only reliable way to prevent them is to keep the two apart in spring, or to collect the eggs, which the adults eat anyway. Is my goldfish breeding? covers what a spawn looks like, and in a pond it is usually over before anyone notices.

Where each one came from

The goldfish, Carassius auratus, was domesticated in China from a wild carp of the genus Carassius, a drab olive fish related to the crucian and gibel carps, first as golden fish kept in temple and garden ponds and later, over centuries of selection, as the fancy breeds. It has always been a fish of bowls, basins and small ponds, and the fancy breeds are the product of keeping it close at hand. The domesticated goldfish tells that story in full.

The koi is a domesticated carp, usually traced to the Amur carp, which some authorities name Cyprinus rubrofuscus and others file under the common carp, Cyprinus carpio; studies of its mitochondrial DNA suggest it may have more than one origin. Its colours were first bred in the mountain villages of Niigata, in northern Japan, in the early nineteenth century, from carp raised in rice-paddy ponds as winter food. Nishikigoi, “brocaded carp”, became a hobby of its own in the twentieth century, built around large ponds and fish judged from above.

The two traditions met in the same ponds, and occasionally in the same fish. Niigata’s koi keepers are also where the tamasaba comes from, a round-bodied goldfish hardy enough to share their cold outdoor water.

Keeping koi and goldfish together

The two share water, temperature and food, and in a pond they get on. What decides whether it works is the pond, and it has to be built for the koi.

The pond. A koi is a 60–90 cm fish, and a 70 cm koi weighs around six kilograms, ten times or more the weight of a large comet. Most koi keepers treat about 4,000 litres (roughly 1,000 US gallons) and a metre of depth as the floor for a few koi, and more is better. A pond that suits goldfish is usually far too small for koi; building a goldfish pond covers sizing from the goldfish side.

Which goldfish. Single-tails only: shubunkins, comets, commons and wakins. They are quick enough to get their share of food and robust enough for a pond’s winters. Fancy goldfish are too slow to compete with koi, too easily pushed around, and many of them do not handle the cold or the currents of a koi pond. The tankmates guide sets out the same rule for every other fish.

Feeding. Koi eat hard and fast, and they take the floating food first. Feed in more than one place, add some sinking food for the goldfish working the bottom, and watch that the smaller fish are actually eating rather than assuming it. Feeding through the seasons covers how much a pond needs as the water warms and cools, which is the same for both.

The plants and the bottom. Koi dig and uproot far more than goldfish, and graze soft plants hard. A pond planted for goldfish will be rearranged.

Waste. Several kilograms of koi produce many times the waste of the same number of goldfish. A koi pond’s filtration is built for that, and goldfish benefit from it; the reverse, a goldfish pond’s filtration with koi added, is where most mixed ponds go wrong.

Through the year. Both overwinter outdoors, but koi need the depth more: water a metre down stays above freezing when the surface ices over. The routine is otherwise the same for both, and a pond through the year walks through it season by season.

Disease crosses over, mostly

Koi and goldfish share nearly all their parasites and bacteria. White spot, flukes, anchor worm, fish lice and the common bacterial infections pass freely between them, and a new fish of either kind can bring them into a pond that has been clean for years. Anchor worm and fish lice are the ones pond keepers see most, because they arrive on new fish and on birds’ feet.

The viruses are different, and they are the reason koi keepers are careful:

• Koi herpesvirus kills koi and common carp, often in large numbers, in the mild water of spring, summer and early autumn. It is not known to make goldfish ill. But the virus has been found in goldfish kept with infected koi, and there is evidence that goldfish can pass it on, which is why serious koi keepers quarantine new goldfish as well as new koi.
• Goldfish herpesvirus attacks goldfish and does not affect koi.
• Spring viraemia of carp can infect both, and it is a notifiable disease in many countries.

So quarantine every new fish, of either kind, for four weeks or more before it joins the pond. Quarantining a new goldfish covers how; a separate tank and patience are most of it. Goldfish diseases explained covers the families of disease behind these names, and a sudden die-off of several fish at once in a pond with koi is a reason to call an aquatic vet the same day.

If you have a koi in a tank

It happens easily: a small fish from a mixed tank, a gift, a fish won or rescued, and a pair of barbels noticed weeks later. The fish is not in danger yet, and nothing has to be done in a hurry, but the tank is a waiting room.

A koi can pass 30 cm in two or three years in good conditions, and it will not stop growing because the tank is small; it will stay stunted, sick or both instead. How big do goldfish get explains stunting and why it is not a way of keeping a fish small, and all of it applies to koi with more force.

The right home is a koi pond. Koi clubs, koi dealers and pond keepers take in small koi regularly, and rehoming a goldfish covers how to find a good home for a fish you cannot keep. What must never happen is release. Common carp is on the IUCN’s list of the world’s 100 worst invasive species, and a koi is a carp; goldfish spread by stocking, bait and pond escapes as well as aquarium release, and a pet koi released into a local pond adds to that.

Before you buy

A few checks decide most of this before it becomes a problem.

• Look at the mouth of the fish you are buying, not the tank’s label. Barbels mean koi.
• Decide on the fish from the water you have. A tank means goldfish, and a koi pond means koi, single-tailed goldfish, or both.
• Plan the pond for the largest fish. Koi reach their size whether or not the pond was built for it.
• Keep fancy goldfish apart. They belong in a tank or a quiet pond of their own, never with koi.
• Quarantine everything. Every new fish, both kinds, every time.

If you are still not sure what the fish is, the identification key starts from the fish in front of you. The first thing it asks about is the back, and a fish with barbels has already answered the question before that.