Maybe you’ve seen spotted koi and goldfish living together, or maybe you’ve seen them in tanks next to each other in the pet store and noted the similarities in their appearances. Many people confuse koi and goldfish with each other, so it’s not surprising that many people also believe that goldfish and koi can breed together.
If you’ve ever wondered, the simple answer is, yes: koi and goldfish can breed together. But keep reading to learn all the facts. 
What are Koi?
Koi, also called nishikigoi, are large ornamental fish that were bred from the Amur carp. The scientific name of koi is Cyprinus rubrofuscus, although you may also see them referred to as Cyprinus carpio. These fish can get quite large and tend to be larger than goldfish, with koi reaching between 36–52 inches in length, while goldfish tend to top out between 12–18 inches. Koi have two sets of barbels on their face, which are the whisker-like sensory organs you commonly see on catfish. These organs are primarily used to find food, especially in low-visibility environments.
What are Goldfish?
Goldfish are a domestic fish that are descendants of the Prussian carp. The scientific name of the goldfish is Carassius auratus. These fish have been selectively bred to create dozens of shapes, sizes, and colors, making goldfish some of the most diverse and underrated fish you’ll spot in a freshwater aquarium. Goldfish lack the barbels that are seen on koi.
Can Koi and Goldfish Breed Together?
So, we’ve established that koi and goldfish are two different species, so they shouldn’t be able to breed together, right?
Wrong!
Since koi and goldfish are both specially bred varieties of carp, they actually can breed with each other, creating hybrid babies. Koi and goldfish breeding with each other is comparable to horses and donkeys interbreeding to create mules, or wolves or coyotes interbreeding with domestic dogs, creating canine hybrids.
Koi and goldfish hybrids are often referred to as “koimets” because they are a combination of a koi and, most often, a comet or common-type goldfish. This isn’t because koi and fancy goldfish can’t breed together, because they absolutely can. It’s because it’s unusual for koi and fancy goldfish to be kept in the same pond because koi are much larger and faster than fancy goldfish, and they have a reputation for bullying fancies.
How Can I Identify Koi and Goldfish Hybrids?
Goldfish and koi hybrids often have one set of barbels, which is a 50/50 split between the goldfish parent having none and the koi parent having two sets. The presence of a single set of barbels is usually the easiest way to determine that you are looking at a hybrid of both a koi and a goldfish.
These hybrids tend to be somewhere in between the sizes of normal koi and goldfish, so you may end up with a fish that is larger than a large goldfish but smaller than a normal-sized adult koi. Hybrids of these two fish also tend to have fins that are more rounded than those of either parent, and they usually have a tail fin that lacks the deep “v” shape that both koi and goldfish have in their tail fins. Thanks to the way some genes express in offspring, you may end up with offspring from the same spawning that have very different characteristics. They can vary significantly in size, color, and behavior.
What Do I Need to Know About Koi and Goldfish Hybrids?
Like many interspecies hybrids, koi and goldfish hybrids are sterile. This means they cannot reproduce at all, even with other hybrids. It doesn’t matter if the hybrid is male or female either. Both sexes will be sterile and, as far as studies have shown thus far, there are not exceptions to this rule.
The behaviors that these hybrids exhibit varies between individual fish, so remember that you may end up with koi-sized fish that are bullies toward the goldfish within the pond. You also may end up with very social, intelligent fish that learn to identify specific, favorite people, much like how goldfish do.
Final Thoughts
Did finding out that koi and goldfish can breed together surprise you?
Even though both fish look alike at first glance, if you study the appearance of a koi and a goldfish, you’ll see they are quite different fish. This may make it seem unusual to consider them crossbreeding, but if you have koi and goldfish that breed together, you may end up with some incredibly unique, beautiful fish as a result.
Unfortunately, these fish will not go on to breed. To develop a population of these hybrid fish, you will have to maintain a population of both goldfish and koi. The good news is that both types of fish can be very long-lived, so their hybrid offspring are likely to be with you for a long time with proper care as well.
If you're new to the world of goldfish keeping or are experienced but love to learn more, we highly recommend you check out the best-selling book, The Truth About Goldfish, on Amazon. From diagnosing illnesses and providing correct treatments to proper nutrition, tank maintenance and water quality advice, this book will help you to ensure your goldfish are happy and to be the best goldfish keeper you can be.
- Next on your reading list: World’s Most Expensive Koi Fish Sells for Millions of Dollars
Featured Image Credit by: Aleron Val, Shutterstock
