All About Koi Fish as Pets (2024)

Koi are a common backyard pet fish species kept throughout the world. These colorful, "brocaded" carp come in a wide range of color varieties and add beauty to backyard landscapes. Koi fish can grow very large, so planning for their entire lives is key to having them thrive for many decades. Here's all you need to know about caring for your koi fish.

Species Overview

Common Names: Japanese koi, Koi, koi carp, Nishikigoi ("brocaded carp" in Japanese)

Scientific Name: Cyprinus rubrofuscus

Adult Size: 2 to 3 feet

Life Expectancy: 30 to 60 years

Pond LevelAll levels
Minimum Pond Size250 gallons
pH6.5 to 9.0
Hardness>100 mg/L
Temperature33 to 85 F (1 to 29 C)

Origin and Distribution

Koi have a long and distinctive history as pet fish. Koi originated from the Amur carp (Cyprinus rubrofuscus) which was kept throughout Asia as a food source. Carp can grow quite large very quickly and are hardy fish, making them ideal food sources. However, natural mutations in color occurred throughout countless generations. These fish were separated from the main population and bred together. From cross-breeding these color variations over centuries, the koi we know today emerged.

These ornamental fish were bred with a scaleless carp imported into Japan from Germany and spawned a mutation of scaleless koi. Known as "doitsu," or "German" in Japanese, these koi are very popular due to the shiny smooth appearance of their skin. They may have a few scales, called a "zipper," along both sides of their dorsal fin, haphazardly placed across their body, or just a few overlapping giant scales on their sides.

Some koi, known as butterfly or long-fin koi, have elongated fins throughout their body. These fins are prone to tears and scarring, so be sure their environment has limited snags to preserve their flowing fins.

Koi are found today throughout the world. There are high-status markets in Japan that carry the world's most ornate and expensive fish, selling for thousands of dollars each! Many koi kept today are domestic-bred koi that do not carry a high price tag but are beloved pets, just like any other fish.

Colors and Markings

Koi have numerous classifications and color specifications. There can be many minute differences in color that move fish between one class or another. Here are some of the more popular colorations.

The Gosanke (The Big Three)

Comprised of the kohaku, sanke, and showa, these koi are the most prized and most valuable throughout the koi hobby. They are often the top show winners at koi shows across the globe.


The kohaku variety of koi is a white body with red pigmentation. There are many patterns within the kohaku variety that contribute to their worth. For classic kohaku koi, the red or "hi" pigment must be deeply saturated and conform to the scale edges, creating a sharp contrast in white and red areas. It is best if all the red pigment is joined in bands or large spots across the body without random scattering of pigment.


Sanke means "tri-colored," and is a kohaku coloration with black or "sumi" marks. There should be no black marks on the head, but red pigment is okay. It is preferred if the fins have some black stripes throughout for contrast.


The showa koi is similarly related to the sanke. Showa koi possess the same coloration of white, red and black, but showa koi have more black pigment than red, creating a vibrant contrasting pattern. The black can extend onto the head and on the bases of the fins.


A bekko koi can be many different body colors with black marks along their backs. A Shiro Bekko has a white body, a Ki Bekko has a yellow body, and an Aka Bekko has a red body.


This collection of koi varieties is divided further still. This group contains the Shiro Muji (pure white), Karasugoi (all black), Goshiki (a mix of all five colors—white, red, black, blue, and dark blue), Chagoi (brown), Benigoi (all red), Kigoi (all yellow) and many others.


The most important aspect of keeping fish in outdoor ponds is that they are subject to the elements and outdoor temperatures. You can try to heat your pond, but if your heater ever fails, warmer fish species will get sick very quickly. It is not recommended to keep tropical fish, includingthe plecostomus, in outdoor ponds in cold weather.

Additional Koi

Before filling your pond to the brim, keep in mind that you will require at least 250 gallons of water per koi. Sure they may not need this when they are small, but when they grow up, they sure will need the extra space. Pregnancy, breeding female koi should have 500 gallons each in order to have enough energy to rear their egg masses. Koi are communal fish, so at least 3 koi per pond are preferred.


It is a complete myth that koi and goldfish cannot be kept together. They are essentially carp "cousins" and share all the same diseases. Goldfish do not need as much room as koi, so some ponds may be better as goldfish ponds, rather than stunting koi in a tiny pond.

Hi-Fin Sharks

A newer addition to the fish hobby, the High-Fin Shark (Myxocyprinus asiaticus) do well in outdoor ponds. Another carp cousin, these fish are more herbivorous, so they may help cut back on some of the algae in your pond, but they certainly prefer koi pellets. These fish can also grow very large and their dark color makes them harder to see in the pond.

Aquatic Turtles

Turtles can be added to ponds with extreme caution. Some turtles get along with fish and do not cause any issues. Naughty turtles will bite koi on their fins and operculum, causing serious damage and disease.

Koi Habitat and Care

The biggest consideration in building and maintaining a koi pond is having enough room for all your fish. Although they start very small, most koi can grow over 24" long in a few, short years. Many owners make the mistake of overstocking their pond when their fish are small and then have to get rid of fish when they outgrow their surroundings. Koi shouldneverbe kept in a tank unless the tank is at least 500 gallons or more.

All koi ponds must have at least 250 gallons per fish. This may sound ridiculous when your fish is only a few inches, but they'll need it when they grow up. More water will always make maintenance easier and keep your fish healthier.

Koi ponds can vary widely in temperature. Koi can tolerate a wide range of temperatures but are easily stressed when temperatures change very quickly. Insulate the sides of your pond, dig your pond to a depth of 24"-48” or more, and provide shade cover to protect your fish from sudden temperature fluctuations.

Since koi ponds are typically kept outside and host very large fish, your filtration system must be well-planned and correctly installed. There are three different types of filtration common in koi ponds: mechanical, chemical, and biological.

Can I Keep My Koi Fish Inside?

Mechanical Filtration

This filtration is responsible for removing large particulates from the water. Many ponds skip these features, which can lead to debris build up in your biological filtration and poor performance. These elements may include:

  • Settling tanks
  • Sieve filters
  • Skimmers
  • Mesh or floss

Biological Filtration

These filters house your good bacteria responsible for running yournitrogen cycle. They require good water flow through the media to keep the beneficial bacteria well oxygenated.

  • Pressurized bead filters (most common)
  • Matting or strapping
  • Gravel or lava rocks

Chemical Filtration

Typical carbon filters are not found on most koi ponds. Many koi ponds will utilize UV sterilizers to kill algae in ponds. UV sterilizers do NOT affect any bacteria or parasite living on your fish.

Koi Diet and Feeding

Koi, like their goldfish cousins, are bottom-feeding omnivores. They eat a lot of bugs and spend lots of time foraging in the substrate. Most koi diets are floating diets, allowing owners to see their fish during feeding time, and most koi adapted well to feeding at the surface. Many owners will see their fish nibbling on algae and assume they like their veggies, but this is not the case. Bugs and bug larvae take up residence in algae and this is much tastier for your fish than boring green algae. Koi are omnivores, though, and will eat lettuce and other leafy greens added to the pond.

There are many commercial koi diets available, and price has no correlation to a food being "better" than any other. Many koi diets are sold for specific seasons, but you don't have to switch your koi diet each season. Store all fish food inside in an airtight container and replace it every 6 months to ensure the water-soluble vitamin content, including vitamin C, stays within healthy levels.

Given their outdoor status, koi appetites will vary depending on their water temperature. In some climates, koi may not be interested in eating at all or eating very little for months during cold weather. This is a normal behavior and is not any cause for alarm. As temperatures warm, fish will return to their normal appetites.

Feeding Your Aquarium Fish the Right Type of Food

Gender Differences

The external differences between male and female koi can be subtle. Fish that are kept in overcrowded conditions or not fed properly may not develop mature gonads. Female koi tend to have rounder heads and bellies, whereas male koi have pointed heads and more slender bodies. Males can be mistaken for females when they are overfed and become obese.

Behavioral differences may also be noted by a keen observer. During the spawning season, typically in the late spring, male koi will chase female koi around their pond. Immature fish may get in on the action simply to imitate the larger fish but are not actually reproductively mature.

Breeding the Koi

Many koi spawnings will occur accidentally. Given proper nutrition and environment, even newbie koi owners can successfully spawn their fish. If your fish do not spawn, that is okay! The females will resorb the mature eggs and go about life as normal.

Most koi spawnings require a destination for eggs to land, such as a plant or artificial spawning brush. With ideal temperatures and nutritional support, koi will spawn in a lively, and at times violent, event. Eggs and sperm are sprayed everywhere, with special attention put on any plants and brushy items along the edges of the pond.

Many times, the only signs of spawning will be a foamy layer on top of the pond and your fish will not be acting normally. Eggs are very hard to see, as they're clear and the size of the top of a pin. Many of the eggs will be eaten by the fish and other invertebrates in your pond, not to mention all the additional eggs that will get sucked up in your filtration. Plan on a large water change after a spawning event since lots of protein-rich reproduction products like eggs and sperm can cause an ammonia spike.

After spawning, plants and brushes containing eggs should be removed from the pond and put in special breeding tubs with excellent filtration and low-flow water. Larger koi have been known to accidentally eat baby koi that get in the way during feeding time. After hatching, baby koi "fry" should be fed a high-protein, high-fat diet for proper development. Once they are a few inches long, they can be moved back to the main pond.

More Pet Fish Species and Further Research

If you like koi, here are some additional species to check out:

  • Comet Goldfish Species Profile
  • Betta (Siamese Fighting Fish) Species Profile
  • Mosquito Control Fish

Check out additional fish breed profiles for more information on other freshwater fish.

How to Treat Carp Pox in Koi

Insights, advice, suggestions, feedback and comments from experts

Koi fish are a popular backyard pet fish species known for their vibrant colors and beauty. They can grow very large, so it's important to plan for their entire lives to ensure they thrive for many decades. Here's all you need to know about caring for your koi fish:

Species Overview

  • Common Names: Japanese koi, Koi, koi carp, Nishikigoi ("brocaded carp" in Japanese)
  • Scientific Name: Cyprinus rubrofuscus
  • Adult Size: 2 to 3 feet
  • Life Expectancy: 30 to 60 years
  • Family: Cyprinidae
  • Origin: Asia
  • Social: Peaceful
  • Pond Level: All levels
  • Minimum Pond Size: 250 gallons
  • Diet: Omnivore
  • Breeding: Egglayer
  • Care: Beginner
  • pH: 6.5 to 9.0
  • Hardness: >100 mg/L
  • Temperature: 33 to 85 F (1 to 29 C) [[1]]

Origin and Distribution

Koi fish have a long and distinctive history as pet fish. They originated from the Amur carp (Cyprinus rubrofuscus), which was kept throughout Asia as a food source. Over time, natural mutations in color occurred, and these fish were separated from the main population and bred together. Through cross-breeding and selective breeding, the koi we know today emerged. Koi fish are found throughout the world, and there are high-status markets in Japan that sell ornate and expensive koi fish for thousands of dollars each. Many koi kept today are domestic-bred koi that are beloved pets [[1]].

Colors and Markings

Koi fish have numerous classifications and color specifications. Some of the more popular colorations include:

  • The Gosanke (The Big Three): Comprised of the kohaku, sanke, and showa, these koi are the most prized and valuable in the hobby.
  • Kohaku: White body with red pigmentation.
  • Sanke: Kohaku coloration with black or "sumi" marks.
  • Showa: White, red, and black coloration with more black pigment than red.
  • Bekko: Different body colors with black marks along their backs.
  • Kawarimono: A collection of koi varieties with various colors, including white, red, black, blue, and dark blue [[1]].


When keeping koi fish in outdoor ponds, it's important to consider their compatibility with other fish and animals. Some important points to note are:

  • Goldfish: Koi and goldfish can be kept together as they are essentially carp "cousins" and share the same diseases. However, goldfish do not need as much room as koi, so some ponds may be better suited for goldfish rather than stunting koi in a small pond.
  • Hi-Fin Sharks: High-Fin Sharks, also known as Myxocyprinus asiaticus, are carp cousins that do well in outdoor ponds. They are more herbivorous and can help control algae in the pond.
  • Aquatic Turtles: Turtles can be added to ponds with caution, as some turtles may bite koi on their fins and cause damage and disease [[1]].

Koi Habitat and Care

When building and maintaining a koi pond, it's important to consider the following:

  • Sufficient Space: Koi can grow over 24 inches long in just a few years, so it's crucial to have enough room for all your fish. Each koi fish requires at least 250 gallons of water, and it's recommended to have at least 3 koi per pond.
  • Temperature Considerations: Koi can tolerate a wide range of temperatures but are easily stressed by sudden temperature fluctuations. Insulating the sides of the pond, digging it to a sufficient depth, and providing shade cover can help protect the fish from temperature changes.
  • Filtration: Proper filtration is essential for maintaining a healthy koi pond. There are three common types of filtration used in koi ponds: mechanical, biological, and chemical. Mechanical filtration removes large particulates, biological filtration houses beneficial bacteria, and chemical filtration may involve the use of UV sterilizers to control algae.
  • Feeding: Koi are bottom-feeding omnivores and eat a variety of foods, including bugs, bug larvae, and commercial koi diets. Floating diets are commonly used, and koi may also eat lettuce and other leafy greens. Feeding habits may vary depending on water temperature, and it's important to store fish food properly to maintain its nutritional value.
  • Gender Differences: Male and female koi can have subtle external differences, such as the shape of their heads and bodies. Behavioral differences may also be observed during the spawning season, with males chasing females.
  • Breeding: Koi can spawn successfully with proper nutrition and environment. Spawning typically requires a destination for eggs to land, such as plants or artificial spawning brushes. After spawning, eggs should be removed from the pond and placed in special breeding tubs for proper development [[1]].

These are the key concepts related to caring for koi fish. If you have any specific questions or need further information, feel free to ask!

All About Koi Fish as Pets (2024)


All About Koi Fish as Pets? ›

Young koi can be kept in an indoor aquarium, but koi grow very quickly and will eventually need a larger aquarium or tank. Many koi owners create beautiful backyard ponds to enjoy their colorful pets. Ponds require ongoing maintenance but provide hours of peaceful enjoyment.

Are koi fish good pets? ›

These fish live long, fruitful lives and make great pets.

Do koi fish recognize their owners? ›

Koi are not only affectionate but intelligent.

They recognize their owner, come when called and can be taught tricks like ringing a bell for food.

Do koi fish like to be petted? ›

Not only can koi fish get to know you but they can also follow you as you come close with food and make interactions creating a special bond. Petting and hand feeding koi fish allows you to engage with these vibrant and graceful swimmers in a whole new way, while also enjoying a calming and therapeutic experience.

What are the cons of koi fish? ›

Ponds with koi require more care than regular garden ponds. The water needs to be kept clean and clear, and the koi fish need to be fed regularly. In addition, koi are more vulnerable to predators than other types of pond fish. You will need to take steps to protect them from cats, birds, and other animals.

Is it OK to touch koi fish? ›

Remove all jewelry that might scratch or injure the fish. Wash your hands with dechlorinated water before touching the koi to protect its slime coat. Relax. Koi can sense if you are very nervous when trying to handle them.

Is it safe to touch koi? ›

A lot of “experts” will tell you to avoid handling your koi as much as possible, and so many koi owners will avoid netting their fish unless there is a situation where it becomes unavoidable.

What makes koi happy? ›

A happy Koi fish has adequate room (not overcrowded with other fish), well oxygenated water, a diverse environment with places to hide (bubbles, waterfall, plants, etc.), clean water (correct filter), and is fed with quality food.

Do koi get aggressive? ›

Stress can cause koi to become aggressive, just like with humans. It can also weaken resistance to diseases and increase the time required to recover from an ailment.

Do koi fish bites hurt? ›

A koi fish bite will only tickle you, but their little nibbles help them eat food from the bottom of your pond and can be dangerous to other fish.

What is the lifespan of a koi fish? ›

Koi have an average lifespan of 40 years. It is believed that the oldest-known koi lived to be nearly 230 years old; the age was determined by testing the fish's scales, which produce growth rings much like a tree.

How long do pet koi fish live? ›

How Long Koi Fish Live In Koi Ponds. Koi are beautiful and long-lived fish, living up to and past 100 years in optimal conditions. In domestic ponds, they typically live around 15 to 30 years. Japanese koi often live 40 years or longer. The oldest koi fish on record was around 200 years old!

Do koi carry diseases? ›

Ich (Ichthyophthirius Multifiliis) is one of the most common diseases among koi fish. It's caused by a parasite that attacks both skin and gills, causing white spots across the body of the fish. It's important to note that Ich is highly contagious, so if you spot it in one fish, it's likely already spread.

Why can't we eat koi fish? ›

Koi fish are classified as bottom-feeders, often recommended to avoid for food. As contaminants like PCBs and Chlordane are usually found on the bottom of lakes and rivers (Such as the Kansas Department of Health and Environment advises anglers to avoid eating carp). The chemicals get stored in the fish's fat.

Do koi get sick easily? ›

Koi is one of the toughest and most resilient fish that rarely gets sick. But this only happens if they are kept in ideal conditions. Keep them in less than ideal conditions, and you will find them getting sick frequently and developing Koi fish diseases.

Are koi fish easy to keep? ›

Is it Hard to Take Care of Koi Fish? No, koi fish care is not hard, but has specific challenges. Keeping their water clean, balanced and aerated is the highest priority. They can survive through winters and are omnivores with plenty of food options.


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