I get this question a lot. “Is my fish a koi or a goldfish?”It’s something I’ve wanted to make a detailed post on for some time, and today is the day. It is hard to confuse fancy goldfish with koi due to their drastically different body shapes.
Slim-bodied fish (i.e. Commons, Comets, Shubunkins) might be a bit more tricky. That’s why I’ve put together this guide to koi and goldfish differences. Some of these methods are more reliable than others. The easiest way to tell the difference between koi and goldfish is by checking their mouths for two sets of short “whiskers.” If these whiskers are present, it’s a koi; otherwise, it’s a goldfish. Generally speaking, of course. That’s why I have 7 main differences laid out to help you identify your fish.
All combined, it should help to paint the picture of the fish you are trying to identify. I hope it helps the next time you’re looking in your pond!
Visual Differences: Koi vs Goldfish
At a Glance
Goldfish are usually far smaller than Koi Carp and can be seen in a much wider variety of body, fin, and tail shapes as well as sizes. The most obvious visual difference between the two is the presence of barbs on the Koi Carp, a trait that the Goldfish is lacking.
Another telling difference is in the fins: the Goldfish has a split tail fin and detached dorsal fin, whereas the Koi Carp’s dorsal fin is attached all the way down its body. Koi also tend to have a far wider range of colors and patterns than Goldfish.
Koi Carp Overview
Koi Carp is a colored variety of the Amur Carp and have been kept for decorative purposes for centuries. Koi Carp are not a separate species from the common carp, and in fact, the word “Koi” is the Japanese and Chinese translation of the word “carp.”
The breeding of these beautiful fish began in Japan in the early 19th century, and they were bred and selected for their unique colors and patterns.
Koi Carp are highly intelligent creatures that cat be trained to get to know their owners and even eat out of their hands! They are an important part of Asian culture and are revered as symbols of perseverance, endurance, and strength. Gold Koi Carp represents wealth and prosperity, blue serenity, and red positivity.
Breeding Koi Carp may seem relatively easy; simply place a male and female together in a pond and let nature run its course. However, it’s not quite that simple. The problem is that it’s rather difficult to correctly differentiate between a male and female Koi, thus making selecting the individuals for breeding a challenge.
A basic method of telling the two apart is that males are more slender looking, while females have a rounder body, but only through experience will you learn to tell them apart confidently.
Around 3-6 years old is the ideal breeding age for Koi, although they can breed at older ages too, albeit less successfully. During breeding, Koi require a lot more energy and thus will need a lot more food, and you should feed them 3-4 times a day during this time.
Lastly, like humans, Koi prefer privacy when breeding. The process may take some time, so give them ample space and privacy!
Koi Carp are fairly adaptable and hardy fish but still require some specific conditions to stay healthy and happy. Their pond should maintain a temperature of between 74-86 degrees all year round, and so you’ll need a heating system during colder months.
PH balance is also a vital factor and needs to stay between 7.0 and 8.6—anything below or above this may cause poor health or even death.
Water plants are also a great addition to their habitat, including water lilies, hyacinths, and duckweed, and small plants or trees to shade the Koi pond are great to keep them cool too. Koi fish need a lot of space to swim around and need roughly 250 gallons of water per adult to live happily.
You can see why keeping Koi Carp can rapidly become expensive.
Health & Care
Koi fish eat a wide range of foods, from small insects to plants and algae, and will even enjoy an occasional piece of fruit as a treat. Store-bought Koi food is the best option as a staple, and they’ll need to be fed at least once a day. The amount a Koi eats varies widely depending on the season, and they’ll typically eat less during the winter months.
Koi Carp are fairly healthy animals if cared for properly and prone to very few diseases. Most of the diseases they tend to suffer from are usually easily treatable too, and so caring for a Koi is typically easy if their habitat and diet are properly maintained.
Koi Carp need a lot of space, and so are ideally located outdoors in a large pond. This makes them better suited to owners that have the space in their yards to house them. These fish do not do well alone or in small tanks and so are not suited for small apartment living. They make a great addition to an outdoor environment in the home where they have plenty of space to swim and plenty of shade.
Goldfish are descendants of the Prussian Carp and so have some shared history with the Koi Carp. That said, they are a wholly separate species and have only a small resemblance to their carp ancestors. Goldfish come in all shapes and sizes due to selective breeding and can vary widely in colors, fin styles, and eyes.
Most commercial goldfish are suited to indoor living only as they are fairly sensitive, but some variations do well in outdoor ponds too.
Goldfish breeding is fairly complicated and not a very easy task. Goldfish need specific temperature changes to induce breeding. Living in the wild, they will usually breed in spring, so you’ll need a temperature-controlled tank to breed them successfully.
Differentiating males and females can also be a challenge, and you’ll typically need to wait until they reach full maturity, about a year old, before you can sex them.
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Goldfish require a lot of tank space to stay healthy and happy, about twice the amount of tropical fish, and prefer warmer temperature water too. In the wild, they prefer slow-moving, calm water and enjoy loads of aquatic plant life. They are fairly easy to look after, and a large tank filled with plants is ideal. Contrary to popular belief, a small fishbowl just won’t cut it!
We recommend a tank of at least 20 gallons per fish, and you’ll naturally need a larger tank if you plan on adding more fish. Goldfish produce a fair amount of waste as they are almost constantly eating, and so adequate filtration is essential. They need a PH of between 6.0-8.0 and prefer still water.
Health & Care
Goldfish have large appetites and are almost always on the lookout for food, but still, for the most part, Goldfish are largely overfed. Commercial Goldfish Flakes are ideal as their primary diet, preferably the floating kind, with the occasional treat of bloodworms or mosquito larvae.
A Goldfish only needs to be fed once a day, preferably in the morning, and you should only feed them what they can eat in about 2 minutes and then remove the rest. With a proper diet and well-maintained tank, a Goldfish can live for up to 15 years, and so they are generally a healthy pet.
The biggest factor to look out for is stress, as this can often lead to illness and lack of appetite. Make sure their tank is clean, warm, and that the PH balance is good, plus keep them in a calm and serene environment to reduce stress.
Goldfish are an ideal pet for owners that live in small homes or apartments that don’t have the space for an outdoor pond. They are fairly low maintenance and are generally healthy animals that live long lives and are a great choice for families with children.
The 7 Key Differences Between Koi and Goldfish Are:
1. Presence of a Pair of Barbels vs. None
This is probably the fastest and easiest way to tell the difference between a goldfish and a koi. Check the mouth for two pairs of short pointy “whiskers” on each side (one pair will be noticeably larger than the other).
If the fish has them, it’s definitely a koi. If not, it’s a goldfish. What are these little barbels anyway? Some speculate that they help the fish navigate through and orient themselves in turbid waters. Kind of like how an insect or snail uses its antennae.
They give the koi a very unique look.
2. Attached Dorsal Fin vs. Detached Dorsal Fin
Now you can look at the end of the dorsal fin closest to the tail. Does it lead straight down to the body? Or does it kind of go under and separate a bit first before joining the fish’s back?
Attached = Koi.
Detached = Goldfish.
This method might be a bit tricky to use until you practice it enough.
3. Flat Under Jaw vs. Rounded Under Jaw
There’s something about the head shape of a koi that is quite distinct from a goldfish’s head. One big reason for that is the flat jaw along the underside of the koi’s head. A goldfish has more of a rounder curve below the chin before the head joins to the body.
Why this is? I have no idea. It seems it is just another thing that is unique to each species and gives them their own special appearance.
4. More Body Weight in Front of Dorsal Fin vs. More Body Weight Behind it
Koi have a larger percentage of their muscle mass in front of the leading edge of their dorsal fins. For most goldfish, it’s more evenly distributed along the body before and behind.
Some older, well-fed goldfish can get quite robust in the area behind their heads between their heads and where their dorsal fins start. But most of them (the slim-bodied ones anyway) are pretty tube-shaped.
Typically, Koi have more of their bulk nearer to their head.
5. Presence of Fancy Features (for Goldfish)
This isn’t always reliable, depending on the variety of goldfish you are looking at. But one thing that makes some goldfish different than koi is that they can have double tail fins. This can indicate the fish like a Wakin, Fantail, or Jikin.
Other fancy features goldfish have that koi don’t include:
- Pom poms
- Bubble eyes
- Telescope eyes
- No dorsal fins
- Short round body
Although Koi varieties such as the longfin or “butterfly koi” might have extremely long, exaggerated fins, this is not considered a fancy feature like those of goldfish.
6. Certain Colors and Color Patterns
Koi and goldfish can share some similar coloring, so this isn’t always the best method for distinguishing between them. Sometimes it can come in handy.
Many goldfish kept in ponds are usually solid orange, white, or orange and white (also known as sarasa). The Shubunkin goldfish has the body shape of a Comet with longer fins but is a calico coloration of white, black, and orange.
Koi have a huge variety of color patterns and scale types, many of which are not seen in goldfish. Some of them are quite incredible! It is the coloration of a koi that often determines its value.
The same is true of the more expensive goldfish breeds on the market. The more rare the color, the higher the price.
7. Much Larger Size Potential vs. Not as Large
Your fish have to be adults to use this method. But sometimes it can come in useful. Koi can get MASSIVE—way bigger than a full-grown goldfish in a pond.
We’re talking up to 4 feet long! That’s almost hard to believe until you see it in person. Most Comet goldfish will never get past 14 inches max. So next time you’re looking in a goldfish and koi pond, keep your eyes peeled for which ones look the biggest.
Those might be the koi!
Why are Koi & Goldfish Different Even Though They Are Related?
Yes, koi and goldfish are indeed “distant cousins.” But according to some sources, the reason they can be so different is that their ancestors are actually two different kinds of carp.
The common carp in Japan is said to be the father of the koi, whereas the goldfish originate from the Prussian or Gibel carp (it’s debated among hobbyists). This means though they both come from carp, they were hybridized from different species of carp with different genetic makeup.
However, the two can interbreed to create sterile offspring. (More on that in another post.)
Can Goldfish and Koi be Housed Together?
Despite their differences, slim-bodied goldfish and koi can make wonderful companions. Both are athletic, strong swimmers with good tolerance for cold weather. Fancy goldfish are not as good of an option for either the fast kind of goldfish or koi due to their delicate nature.
More on that in another post as well.
When looking at koi vs goldfish, hopefully, you now are able to identify the fish with ease. Now, I want to hear from you. Have you tried any of these methods to tell the difference between koi and goldfish? If so, I want to hear how it went.
Or maybe you have some tips I haven’t mentioned.