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 🙂

Now:

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.

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!

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 a 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 itself in turbid waters.

Kind of like how an insect or snail uses its antennae.

They give the koi a very unique look.

Related Post: Goldfish Vs. Koi

2. Attached Dorsal Fin vs. Dettached 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

Look:

Koi have a larger percentage of their muscle mass in front of the leading edge of their dorsal fins.

For most goldfish on the other hand…

… it’s more evenly distributed along the body before and behind.

Some older, well-fed goldfish can get quite robust on 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 is another type of pond goldfish 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
  • Wens

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.

That said:

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.

See also: Koi Fish For Sale

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 ones might be the koi!

Why are Koi & Goldfish Different Even Though They Are Related?

Yes, it is true that koi and goldfish are “distant cousins.”

But according to some sources, the reason they can be so different is because 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.

But get this:

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 a good tolerance to 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.

Final Thoughts

Now I want to hear it 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.

Please feel free to share your thoughts in the comments below!

Featured Image Credit: left – freedomnaruk, Shutterstock, right: Napat, Shutterstock