Koi: Fish Species Profile

Characteristics, Origin, and Helpful Information for Hobbyists

Koi carp in a pond

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Koi are a common backyard pet fish species common throughout the world. These colorful, "brocaded" carp come in a wide range of varieties and add to beautiful landscapes. They can grow very large, so planning for their entire lives is key in having them thrive for many decades.

Species Overview

Common Names: Koi, koi carp

Scientific Name: Cyprinus carpio koi

Adult Size: 2 to 3 feet

Life Expectancy: 30 to 40 years

Characteristics

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) 

Origin and Distribution

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

These ornamental fish were taken to Germany and spawned another natural mutation of scaleless koi. Known as "doitsu," or "German" in Japanese, these koi are very common. 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.

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. Most koi kept today are domestic 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.

Multiple koi varieties

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The Gosanke

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.

Kohaku

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 most 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. It is best if all the red pigment is joined in bands across the body without random scattering of pigment.

Sanke

Sanke means "tri-colored," and is a kohaku coloration with minimal 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.

Showa

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.

Bekko

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.

Kawarimono

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

Pondmates

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 heating ever fails, warmer fish species will get sick very quickly. It is not recommended to keep tropical fish, including plecostomuses, in outdoor ponds.

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.

Goldfish

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 container.

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.

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 operculums, 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 up to 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 should never been kept in a tank unless the tank is at least 500 gallons or more.

It is critical that all koi ponds have at least 250 gallons per fish. This may sound ridiculous when your fish is only a few inches, but they will 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 very wide range of temperatures, but are easily stressed when temperatures swing very quickly. Insulating the sides of your pond, digging your pond to a depth of 24" or more and providing shade cover will 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 on koi ponds.

Mechanical Filtration

This filtration is responsible for removing large particulates from the water. Many ponds skip these features, which can lead to 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 your nitrogen cycle.

  • Pressurize 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

There are many commercial koi diets available. Price has absolutely no correlation to a food being "better" than any other. Many koi diets are sold seasonally, but you do not have to switch your koi diet per 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.

In the wild, 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. Most captive koi are well adapted to feeding at the surface. Many owners will see their fish nibbling on algae and assume they like their veggies. 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.

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. This is a normal behavior and is not any cause for alarm. As temperatures warm, fish will return to their normal appetites.

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 obese.

Behavioral differences may also be noted by a keen observer. During 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. Many times, the only signs of spawning will be a foamy layer on top of the pond and your fish will not be acting normal. 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, 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

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

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