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Blue Koi Fish: Info, Pics, Origin & Facts

Sarah Psaradelis

By Sarah Psaradelis

Blue Koi Fish

Koi fish are well-known for their vibrant colors and elaborate patterns, with blue being one of these. Koi have been selectively bred to be kept in ornamental ponds for hundreds of years. While colors such as red, white, and black are more common colors found on koi fish, blue is slightly more uncommon. Certain varieties of koi naturally have a blue-ish grey coloration, such as in the rare Asagi koi.

You will find that a blue koi fish’s care requirements and information are relatively the same as other koi fish. This is because blue is used to describe the koi fish itself, and not a separate variety of koi.

Quick Facts About Blue Koi Fish

Species Name: Cyprinus rubrofuscus
Family: Cyrinidae
Care Level: Beginner-friendly
Temperature: 59 to 77 degrees Fahrenheit
Temperament: Peaceful and social
Color Form: Blue
Lifespan: 25 to 35 years
Size: 20 to 28 inches
Diet: Omnivores
Minimum Tank Size: 1,000 gallons
Tank Set-Up: Freshwater, ornamental ponds
Compatibility: Other koi fish or goldfish

Blue Koi Fish Overview

Nearly all blue koi fish are considered Japanese Koi. These Japanese koi have a lengthy history in China and Japan. It is theorized that Japanese koi originated from carp that developed color mutations. These carp were originally raised by rice farmers in China as a source of food more than 2,000 years ago. These carp soon developed color mutations such as reds, whites, and oranges.

When the Chinese invaded Japan, the carp with the color mutation were brought in. Since the carp’s colors stood out against the typically dull and neutral-colored carp, the Japanese took interest in selectively breeding the carp to produce the vibrant and colorful koi fish we know today. The selective breeding of the color-mutated carp dates to the early 1800s, but it wasn’t until the early 1900s that the koi became noticed.

A Tokyo exhibition took place in 1914 which was the start of the koi fish’s popularity. An emperor was gifted beautifully colored koi fish for his moat, and this led to the koi becoming recognized for their colors and patterns that weren’t the norm at the time

Since the koi’s selective breeding in Japan, there has been a development of different koi fish colors. This is why koi can be found in colors such as blue, usually with a mixture of different colors like white, black, and red depending on the variety of koi.

How Much Do Blue Koi Fish Cost?

Since most blue koi are the Asagi variety, you can expect these koi to sell for $1,000 to $2,500 as an adult. However, the typical price for pet shop koi ranges from $75 to $200. The blue koi will cost more if they are purchased from a breeder since you can guarantee that the koi is healthy and a quality-bred fish.

Typical Behavior & Temperament

Like most koi, blue koi fish are peaceful and social. They prefer to spend most of their day foraging for food and grouping with other koi fish. It is uncommon to find a koi fish being aggressive, and any aggressive behaviors are usually only seen in males during the breeding season. In a pond environment, koi are slow-moving fish that cluster in groups.

They enjoy the company and safety they feel in same-species groups and can get stressed if they are kept alone or in too small groups. Most koi will become more energized during feeding time since they are food motivated. Since koi aren’t very confrontational fish, they do not harass, nip, or bully other fish and generally keep to themselves.

Appearance & Varieties

The word “blue koi fish” is used to describe a coloration of koi, and not necessarily a variety of koi fish. Even though there are over 100 different varieties of koi, only a few have blue colors. It is very rare to find a koi with a solid blue coloration, and you will only find it as a hue in solid black koi fish.

A Japanese koi fish known as Asagi has a characteristically blue or indigo coloration. The blue can be found in a netting-like structure on their backs, although it can appear grey or purple in certain lighting. These Asagi koi will also have reds on their fins or bellies, and they have a whitish-grey base color.

Since blue koi are the Japanese varieties, they all have the opportunity to reach a length of 36 inches. This is quite big for a fish, and one of the many reasons koi require such a large pond to live in. However, most Japanese koi will only grow around 20 to 28 inches in size when kept in standard garden ponds. The older and more space blue koi fish have to grow, the larger they can get. In terms of weight, a full-grown adult blue koi fish can weigh between 9 to 16 pounds.

How to Take Care of Blue Koi Fish

Habitat, Tank Conditions & Setup

When caring for blue koi fish, here’s what you need to know.

Tank Size

Small tanks or aquariums are not suitable for blue koi fish, since these fish need at least 250 gallons of water per fish. Instead, you should house blue koi fish in a large and filtered pond. A minimum of 1,000 gallons for four juvenile blue koi fish will suffice, and the larger the pond is, the more koi you can house inside.

Water Quality & Conditions

The water quality is important if you want to keep your koi fish healthy. They need their pond to be cycled and filtered before they are placed inside since the cycling process ensures that there are enough nitrifying bacteria to convert their waste. As large and messy fish, a koi pond can quickly become dirty.

Blue koi require a freshwater setup with a temperature of 59 to 77 degrees Fahrenheit. They aren’t picky with the temperature, and healthy adult koi can handle occasional temperature fluctuations quite well,

freshwater aquascape rimless aquarium with big natural root, live plants
Image Credit: Alex Unders, Shutterstock


You don’t need to use a substrate in your koi fish pond, but if you do, aquarium sand or fine gravel will be fine. Small koi can choke on large pieces of gravel, so avoid placing this type of substrate inside.


Blue koi fish can benefit from live plants in their pond, such as lilies, water lettuce, and floating aquarium plants like hornwort. These live plants help to create a more natural environment for the koi while sheltering them and helping with the water quality.

Rock Flower Anemone with friendly Slug
Image Credit: VIP Reef Aquariums, Shutterstock


Koi are not very fussy about lighting, but they should have a day and night cycle. In an outdoor pond, this cycle follows a natural day and night cycle. However, in indoor ponds, you might need to add an artificial lighting system that gives off a moderate amount of light for 8 to 10 hours per day. Avoid placing the koi pond in an area where the sun constantly shines over it, since this can cause the water too to become too warm for the koi.


A filter is a must-have for koi ponds. They will ensure the water is kept filtered to keep it clean and prevent the water from becoming stagnant. A stagnant pond with poor circulation is going to create a dirty environment for your blue koi fish. The pond should not only circulate the water but also ensure that there is enough surface agitation. This helps move oxygen through the water, which is essential for your koi fish.

aquarium with filter
Image Credit: Dovzhykov Andriy, Shutterstock

Are Blue Koi Fish Good Tank Mates?

As social fish, blue koi need to be kept in groups of ideally three or more. A koi who is kept by themselves can become stressed, which can lead to decreased activity and even sickness. Other koi are the koi fishes’ best tank mates since these large and peaceful fish do best when kept in a pond of their species.

By housing blue koi fish with incompatible tank mates, you risk making both species of fish stressed or living in an unsuitable environment.

If you want to keep other fish with your group of koi in a pond, single-tailed goldfish are the next best option. These fish have similar care requirements and can cohabit peacefully with koi. However, single-tailed goldfish need to be at least 6 inches in size before being housed with koi. This is because larger koi fish can eat any goldfish that can fit inside their mouths.

What to Feed Your Blue Koi Fish

Blue koi fish are omnivores and require pelleted food as their daily diet. The pellets should be formulated for koi fish with a mixture of animal and plant-based proteins, carbohydrates, vitamins, and minerals.

A healthy diet is going to ensure that your koi can maintain a good weight and get all the nutrition they need to function throughout the day. You can feed your blue koi fish once or twice a day depending on their size and how many you have in the pond. Avoid overfeeding your koi though, since the leftover food can contribute to poor water quality.

Keeping Your Blue Koi Fish Healthy

When kept in the right environment and fed a healthy diet, blue koi can live for 35 years and sometimes more. It is easy to keep your koi fish healthy and thriving in their pond or large aquarium.

  • House your blue koi in a large pond over 1,000 gallons in size. This allows ample room for the koi fish to grow, while the larger volume of water helps to dilute their waste.
  • Run a pond filter and the pump continues to circulate the water and keep it filtered.
  • Regularly maintain the pond to ensure that the water quality is good, you can do this by testing the water using a liquid testing kit for ponds.
  • Ensure that the koi are fed a healthy and balanced diet. You can supplement their diet with bloodworms or spirulina for more protein and nutrients.
  • Keep the blue koi fish in freshwater only at a temperature between 59 to 77 degrees Fahrenheit.


Your blue koi fish will be ready to breed at around 3 years of age. The ideal breeding season for koi is from late spring to early summer. During this time, you will notice spawning behaviors between the sexually mature koi. The female koi will lay thousands of eggs at a time that are then externally fertilized by the male koi. You should ideally keep the eggs and fry in a separate pond or tank since the adult koi will eat their young. The eggs will hatch after several days, and the fry will grow quickly on a high-protein diet with good water quality.

Are Blue Koi Fish Suitable for Your Aquarium?

Blue koi are rare fish that will make a great addition to your koi pond. If you are looking for a peaceful-natured fish that looks striking when kept with other varieties of koi fish, then purchasing blue koi fish is something to consider.

Blue koi like the Asagi variety grow up to have fascinating blueish-grey patterns that look spectacular when viewed from above in a koi pond. Most blue koi fish can grow quite large and live for many years, so be sure to keep their environment suitable enough for these fish to thrive in.

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