Interesting & Fun Facts About Koi Fish 2020 (Updated)

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Interesting, Educational & Fun Facts About Koi Fish & Carp 2020

One of the most popular and beloved pond fish species, koi carp have been around for hundreds of years, with early varieties being bred centuries ago. As such, there is a vast array of interesting, historical, and even mythical facts that even the most adoring koi keeper may not be aware of:

1) Koi Carp were Domesticated in 4th Century China

Traditional Chinese architecture and koi pond with native plants

Carp domestication began as early as the 4th century in China, with modern koi breeding originating in Japan and China several thousand years ago. They descend from the common carp (Cyprinus carpio), and are commonly called “Goi” in Japan, which translates to “carp.” It can also mean “love” or “affection,” with koi traditionally being seen as a symbol of friendship, love, and kindness.

2) The Ancestor of Modern Koi is the Common Carp

Two adult common carp, ancestor of koi carp

mtDNA sequencing supports the commonly accepted (but sometimes contested) fact that koi’s ancestor, the common carp, originated in East Asia. Furthermore, these DNA studies found that larger scale carp domestication and breeding to obtain brightly colored varieties (which came to be known as koi) occurred in China in 6,000 B.C. These koi were then often gifted to those with political and economic power, and as such made their way to Europe and, eventually, the Americas.

3) Koi Can Live Well Over 50 Years

Healthy adult taisho sanke koi in clear pH balanced pond water

Koi are able to live longer than 50 years if they receive proper nutrition, water is kept clean, filtered, and within their optimum pH range of 7.5 to 8.0, and they are not stressed or overcrowded. Sadly enough, most koi average less than 10 years as they are not properly taken care of and placed in ponds or tanks that are too small, don’t receive proper food, etc. However, koi can also exceed that 50 year mark, with the oldest koi living to be 226!

4) Over 100 Varieties of Koi Carp Exist Today

Many different healthy koi carp varieties in a well maintained pond

There are over 100 varieties of carp existing within 13 classes, with more still being developed. One of the most popular is the ghost koi, which has beautiful metallic scales and was bred in the 1980’s. Size and shape are all essentially the same no matter the koi, but there is a nearly limitless variety of color, patterns, and scale arrangement.

5) Some Koi Are Very Valuable & Expensive

Expensive and beautiful white, black, and orange koi with great bloodline

Most koi sell for under $50 at pet shops, but serious breeders are sometimes willing to pay tens of thousands of dollars for a single adult koi. The most expensive koi ever sold was valued at 1.8 million dollars due to winning the grand championship at the 2017 All Japan Koi Show.

6) Koi Are Naturally Bottom Feeders

Several adult koi feeding in the sediment at the bottom of a healthy pond


Though you should keep your water well oxygenated and filtered with proper pH and temperature for koi, don’t worry if you have a bit of sedimentation. Koi are naturally bottom-feeders, and in nature are typically found in waters with clay or muddy bottoms as they enjoy rooting around.

7) Koi Will Eat Algae & Insect Larvae  

Adult koi carp eating algae and keeping pond water clean

Following along with the previous statement, as omnivorous bottom-feeders koi are not opposed to eating algae and insect larvae. While they won’t prevent an algae bloom or completely eliminate mosquitos, they can make a dent in the population. They’ll also munch on plant matter, tadpoles, snails, and sometimes fish that are smaller than they are (if they’re hungry enough).

8) Koi Do NOT Just Grow to Tank Size

Koi carp in too small of a tank


A common myth is that koi will only grow to fit the size of the tank or pond that they’re in. This is false – koi will continue to grow until they reach maturity around 10 years of age. If kept in too small of an enclosure, they’ll simply die before reaching adulthood, which fuels the misconception that they only grow to fit the area.

9) Koi Don’t Generally Like to Eat Plants

Several koi carp not interested in eating lily pads growing in a pond

Another erroneous belief is that koi will always eat your plants. However, this is typically only the case if you A) aren’t feeding them enough, and/or B) you only have a couple of plants. Koi are curious and can be opportunistic feeders, so if you only have a handful of plants, they’re much more likely to show interest in them and damage them.

However, if you have a large variety of submerged, floating, and emergent plants, not only will water quality improve (better temperature regulation, natural filtration, and increased dissolved oxygen), but your koi will show less interest since the plants are all over. If your koi do still nibble on the plants, plant death is much less likely as they’ll be feeding a small amount on a higher number of plants, rather than feeding a lot on a couple of plants.

10) Koi are Associated with Bravery & Success in Japanese Legends

Koi symbolizing harmony, bravery, individuality, success, strength

Due to their ability to adeptly swim upstream and even up waterfalls, koi are often associated with bravery, success, strength, and individuality (going “against the current”). Japanese legends tell of a koi that swam up the Yellow River to a waterfall known as the Dragon Gate. Upon resolutely ascending the waterfall, it was transformed into an otherworldly, magnificent dragon, thus reaching enlightenment.

The legend deems that any koi that follows suit will attain the same fate, and holds as a popular metaphor for life. This is another reason why koi were historically gifted to those in power – they symbolize prosperity.

11) You Can Determine the Age of Koi Through Their Scales 

Close up of koi fish scales to calculate age

The age of koi, as with any fish, can be determined by looking at their scales. Each scale possesses rings called annuli, which resemble the growth rings in trees. Generally, summer annuli are wider and correlate with higher temperatures and greater nutrition, while winter annuli are thinner due to cooler temperatures, decreased activity, and decreased feeding.

12) Wild Koi Carp are Now Considered “Vulnerable”

Wild common carp in natural environment threatened by human activities

Wild koi carp (common carp) are considered “vulnerable,” due to loss of habitat, overfishing, and hybridization with domesticated koi. Feral koi (koi that have either been released from or escaped captivity) have become such an issue that they are considered pest fish on every continent except Antarctica, with fishing encouraged to help control their populations. With this in mind, please do not release any of your domestic koi (or any other domestic or non-native fish) into natural waterways, as this can severely damage ecological function and integrity!

13) Koi Actually Have Teeth!

Close up of the teeth found in a koi's throat

Koi have teeth in their throats! They use their barbels, if present, to sort edible from inedible matter, suck the food into their mouths, and then grind it up with their unique throat teeth before swallowing. They also shed these teeth as needed, showing no interest in food for several days and appearing to convulse near the water’s bottom as they work the teeth out of their throats and spit them out. If you have a dark pond bottom, you may be able to see them.

14) Koi Carp Can Get Sunburn

Koi carp with mild sunburn

Koi have incredibly sensitive skin, particularly those with lightly colored or white scales, and are prone to getting sunburned. Most cases are mild, and are observed as a light pink hue that may resemble a rash. To prevent these, make sure that koi are kept in ponds that are a minimum of 3 feet deep, provide plants in and around the pond that shade at least 50% of it, and make sure that your water does not exceed 80° F (~ 26° C).

15) Koi Were Traditionally Passed from Generation to Generation in Japan

As koi are so long lived, Japanese tradition meant that koi were often passed on from generation to generation as a family heirloom and symbol of respect and love. Some households in Japan still hold to this tradition.

16) Koi May Eat Their Offspring During Spawning

Koi are not opposed to eating their own. During spawning, both male and female adult koi will eat koi fry, and as such fry should be kept separate from adults. Presumably, this habit evolved as a method to prevent overpopulation from occurring and to conserve resources, as between natural environmental pressures and cannibalism only a few fry would survive to adulthood.

17) Koi Are Smart Enough to Recognize You

Quite intellectual and curious creatures, koi can recognize their owners and can be trained to eat from your hand, or they may simply pop up inquisitively if they see you by the pond.

18) In Winter Koi Enter a State Called Torpor 

Koi may enter a temporary state of hibernation during the winter, known as torpor. Torpor, unlike hibernation, is short-term and can last from several hours to several weeks. Creatures in torpor are easily roused, while true hibernation often lasts months and puts the animal in such a deep state of slumber that it takes quite a lot to bring them out of it.

Typically, if water temperatures are consistently below 50° F (10° C), koi will enter torpor, in which they will hold very still for extended periods of time, eat significantly less as metabolism and protein synthesis slows, and likely huddle together in groups for warmth.

19) Koi Are Actually a Subspecies of Common Carp

Koi are not truly a different species from common carp, but rather are a subspecies that has been selectively bred for more vibrant coloring and scale patterns. Because of this, if not bred selectively, they will revert back to the duller coloration of common koi within a few generations.

20) Koi Grow 0.5 Inches per Month on Average

Most koi grow an average of half of an inch per month, typically reaching full size somewhere between 8 and 10 years old (growth will of course fluctuate from month to month, slowing down after the first couple of years).

21) The Average Adult Koi Carp Weighs 12 Pounds

The average weight of adult koi is around 12 pounds, with a length between 2 and 3 feet. The largest koi, however, was 4 feet long and weighed 90 pounds! She lived to be 17 years old – fairly long, considering her ample size.

22) Koi Carp Are Very Powerful Swimmers

Koi carp, as mentioned previously, are unbelievably powerful swimmers. They may not showcase this much in your calm pond, but koi that have escaped or been released into the wild have been tracked travelling well over a hundred miles upstream in a span of only a few months in order to spawn and reach better feeding grounds. This is on par with Atlantic salmon.

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