10 Incredible Koi Fish Facts

Japanese Koi in a pond.
tonirichsag1411/Shutterstock.com

Written by Patrick Sather

Updated: August 15, 2023

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Koi fish are some of the most colorful and admired fish in the world. Known in Japan as nishikigoi, or brocaded carp, koi make popular attractions in ponds and gardens. They come in an array of dazzling colors and patterns and are known for their friendly and sociable nature. Some koi are so sought after that they can fetch a high price at auction and cost thousands of dollars. Here is a list of 10 incredible koi facts that demonstrate what makes these fish so captivating. 

Discover 10 fascinating facts about the beautiful koi fish.

10. People Have Bred Koi For Hundreds of Years

Doitsu koi fish swimming among other koi fish- one of the most expensive koi fish

Humans have bred koi fish for nearly 1000 years.

For our first entry on this list of koi fish facts, we’ll take you back to the beginning. Most people think that koi originated in Japan. While it’s true that they have become synonymous with Japanese culture, koi actually trace their history back to several varieties of carp from Central Europe and Asia. Chinese farmers used to raise carp in rice paddies as a source of food and began to selectively breed carp for color mutations nearly a thousand years ago. 

The Niigata Prefecture on the northeastern coast of Honshu is credited with giving rise to many of the koi varieties alive today. Beginning around the 1820s, farmers that used to raise koi for food began to breed ornamental Amur carp for pleasure. Over time, they created more and more varieties and eventually paved the way for the modern, systemic breeding of koi. 

9. There Exists Over 100 Different Varieties of Koi Fish

Beautiful Koi in a pond in Japan.

Koi fish have different varieties.

Historically, carp look rather dull-looking, typically appearing in shades of grey, brown, or black. The farmers and breeders that devoted themselves to selectively breeding carp with colorful mutations changed the way these fish looked completely. Through their efforts, they created red, then blue, then multi-colored koi. They even managed to create koi with different patterns and even metallic-looking koi.

The Zen Nippon Airinkai is a group in Japan dedicated to the breeding and categorizing of koi species. Today, the group officially recognizes over 100 different koi varieties that belong to one of 16 different groups. These range from the red-and-white Kōhaku – one of the first koi developed – to the Kumonryū, a black koi with swirling white patterns that resemble drawings from early Japanese ink drawings. More and more varieties are being created all the time, such as the ghost koi, which was made by breeding Ogon koi and wild carp with metallic scales. 

8. Koi Fish Are Quite Tough

White Fish - Butterfly Koi

Koi fish are famous for their hardiness.

Like their ancestors the carp, koi enjoy a reputation as tough, hardy fish. So long as they receive proper care, they can live for a long time and resist parasites that plague other fish. Although they live in cold water, they thrive in water temperatures between 59 and 77 degrees Fahrenheit. They can even survive outside in the middle of winter, so long as they can get enough oxygen. When temperatures plummet, they slow down, eat less, and remain near the bottom of their pools.

Koi can not only handle extreme environmental changes but also water conditions. While koi enclosures usually require a filter and a heater, koi breeders will attest to the sturdiness of these fish. They are strong swimmers and can swim well against a powerful current and even up waterfalls. Many fish struggle with stress, but koi typically bounce back quickly and easily when introduced to new water conditions. 

7. Koi Fish Can Grow To Enormous Sizes

Koi fish isolated on white background.

The weight of koi fish depends on their variety.

Aside from their color, one of the things that people first notice about koi fish is their size. Koi easily rank as some of the largest species of domestic fish that people commonly keep in ponds. Generally speaking, most adult koi measure between 12 and 24 inches long and weigh upwards of 12 pounds. That said, koi can continue to grow so long as they have enough space, food, and care.

Depending on the variety, koi can reach even larger sizes. The largest koi ever recorded is a jumbo koi named Big Girl. Big Girl was bred by a specialist breeder in Japan and now lives in Wiltshire, England. She measures over 4 feet long and weighs more than 90 pounds. To maintain such a size, Big Girl must eat a minimum of 1 pound of food per day. 

6. Koi Fish Can Live for a Long Time

Koi fish swimming in the pond.

Koi fish have a long lifespan.

The next entry on our list of koi fish facts is also one of the most astonishing. It’s well known that some animals enjoy extremely long lifespans. Parrots can live to be nearly a hundred years old, while tortoises can live over 180 years. While many people know about the incredible longevity of these animals, few know that koi are also exceptionally long-lived. 

Typically, koi outside of Japan live around 15 to 20 years. However, koi in Japan often live for up to 40 years. Still, one koi fish named Hanako proved that – under the right conditions – koi can live exponentially longer. According to researchers, Hanako lived to the ripe old age of 226, making Hanako the oldest koi fish ever recorded. 

5. Koi Fish Will Eat Just About Anything

koi fish food

Koi fish are opportunistic eaters and consume a wide variety of foods.

Koi fish are omnivorous and eat a wide variety of foods. As opportunistic eaters, they will consume just about anything they can fit into their mouths, including other fish. Some of their favorite foods include peas, greens, watermelon, and other fruits. They will also eat bugs, worms, algae, seeds, and other aquatic life. There also exist many types of pedigree koi food made by commercial manufacturers. These formulas contain all of the ingredients necessary for meeting a koi’s nutritional requirements. 

As natural bottom feeders, koi tend to hang out near the bottom of their pools. This can make health checks difficult, as owners need to frequently check on their koi for ulcers, parasites, and disease. To encourage koi to swim near the surface, owners often provide koi with food that floats on the surface of the water.  

4. Female Koi Fish Can Lay Tens of Thousands of Eggs

Baby koi fish, about 1st month after birth, in a fish tank.

Female koi fish lay a lot of eggs.

If left to their own devices, koi naturally spawn either once or twice a year in the spring and summer. During the breeding season, female koi lay clutches of eggs after encouragement by males. Males then swim over the eggs and fertilize them. Within just 4 to 7 days, the young koi known as “fry” will hatch. 

Unfortunately, most koi eggs do not survive long enough to hatch. Many of the eggs perish due to a number of factors, most notably being eaten by other fish, including other koi. However, koi females overcome the odds against facing their offspring through sheer numbers. A single female frequently lays up to 50,000 eggs in a single breeding season. The larger the female, the more eggs she can lay, and especially large females can lay up to 100,000 eggs!

3. Koi Fish Can Recognize Faces

Oldest Fish - Koi Fish

Koi fish are surprisingly intelligent.

People tend to think of fish as rather stupid. In fact, for years, many people believed that fish could not even feel pain. Recent studies fly in the face of this belief and point to evidence that suggests fish are much more intelligent than first imagined. Not only can they feel pain, but some species can also manipulate tools and possess powerful working memory. 

For years, koi owners and breeders have known just how intelligent koi actually are. Koi are naturally curious and can be taught tricks, including taking food straight from your hand. Over time, they grow quite close and loyal to their owners, and evidence suggests they can even recognize faces. 

2. Many Wild Koi Fish Were Raised in Captivity

Koi fish swim in the clear pond and are eating.Many orange and white koi fish in the pond are clear and clean.

While there are wild koi populations around the world but they originated from farms or domestic ponds.

Although koi can trace their lineage back to wild carp, the modern koi we all know today were bred in captivity. However, there now exists a number of wild koi populations around the world. That said, these koi don’t come from the wild. In fact, most if not all wild koi originally came from farms or domestic ponds. Over several generations in the wild, koi eventually lose their fantastic coloration and begin to look like regular carp.

Still, this doesn’t mean that you should go about releasing captive carp into the wild. In some areas, koi are even considered an invasive species. As bottom feeders, koi can make the water murky due to their constant moving around and tendency to disturb aquatic substrate. In addition to making water unattractive, their activity can slowly destroy aquatic habitats and make the environment unlivable for certain animal and plant species. 

1. Koi Fish Are Symbols of Good Luck

Beautiful yellow and black koi swimming in a pond.

In Japan, koi fish symbolizes good luck.

For our final entry on our list of koi fish facts, we’ll end with a piece of cultural wisdom. As previously mentioned, most people tend to associate koi with Japan. After 200 years of selective breeding and care, koi have become a symbol of Japanese culture. In Japan, koi symbolize good luck, prosperity, and good fortune. Furthermore, koi also symbolizes perseverance and patience, much like the dedicated breeders who have devoted years to their care. 


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