Koi Fish: A Guide To Keeping Them In Aquarium!

The Koi Fish (Cyprinus rubrofuscus) is a large fish that was domesticated a long time ago. These beautiful fish were first bred in Japan to create beautiful colorations and patterns. Koi fish are considered to be lucky in Japan and nearby countries and are kept in large aquariums or ponds.

These fish are considered to be the king of pond fishes but they can also live in indoor larger aquariums. While most consider them to be pond fish, you can still grow them in aquariums when they are smaller.

Because keeping these fish in aquariums can be tricky, we made this guide to help you narrow its requirements down and see if your aquarium is a good fit for these stunning fish or not. So keep reading to learn more. 

Species Profile & Overview

Koi Fish were first domesticated in ancient Japan around the time the Jin Dynasty was ruling. These fish were selectively bred from wild carp fish to create stunning colorations. After many hundreds of years of selective breeding, the koi fish look completely different than their wild ancestors. 

The word Koi is an informal name for the Amur carp or Cyprinus rubrofuscus. Koi is a Japanese word that is short for nishikigoi and means “brocaded carp”. In Japan, the koi symbolizes strength, patience, courage, and success through perseverance. This is because carp are known to swim against the current and overcome obstacles. Koi is also a symbol of luck, prosperity, and good fortune in many countries including Japan.

Despite being kept for centuries, the most sought-after types of koi fish are actually created just in the past few centuries. The koi breeders in Japan are still trying to improve the quality of this fish and even trying to create new varieties. 

Koi Fish are technically the larger, more colorful cousins of the goldfish. These fish are typically kept in ponds from the beginning of their domestication. However, you can also keep them indoors in larger aquariums if you really love the fish. These fish can live in large aquariums that have enough length and width to let the fish swim easily. 

Koi Fish is a type of carp fish and can live in streams, rivers, lakes, ponds, or any other body of water around the world. They are very hardy fish and can live outdoors in icy cold winters. This makes them a threat to the native habitat where you are living. This is why you will need to be careful not to accidentally release them in the wild.  

Appearance & Types

Koi Fish In Aquarium

Koi have many different classifications and color specifications that make each variant unique. Though the overall shape of the body is the same, the biggest difference among them is their colors and patterns. These colors and patterns are the many ways to differentiate variants of koi.

These ornamental fish are interbred with a scaleless carp from Germany to create a scaleless variant of koi. These scaleless koi are known as Doitsu which means German in Japanese. Doitsu is a scaleless koi that has shiny smooth skin. They will develop the same beautiful patterns as the regular Japanese koi.

The most famous types of Koi Fish are Asagi, Doitsu, Ginrin, Tancho, and many others. Koi also come in long-fin variants that are known as butterfly koi. While butterfly koi look very beautiful, most seem to show interest in regular short-fin koi over long-fin variants. 

Koi Fish are sexual dimorphism species. The males and females have visible differences but a trained eye is sometimes necessary to differentiate them. The most obvious way of identifying a male from a female koi is by looking at the anal vent of the fish. 

To identify the sex of a koi fish by vent area, the fish has to be mature. Females will develop protruding advent that is visibly out. The males, however, have a smaller vent that is tucked inside. Remember that the vents of young koi are similar in both genders before sexual maturity.

Adult Size

Koi are large fish that grow to about 2 – 3 feet in length. However, in smaller ponds and aquariums, these fish tend to only grow to about 12 – 15 inches in length. The water conditions, diet, and size of the tank has a direct impact on their growth rate. 

In Japan, breeders keep these fish in large mud ponds where they get natural foods and are exposed to natural environments. This conditions the koi to grow very fast and very large. In recreation ponds and larger aquariums, this fish will stay smaller and can not grow to its max 3 feet length.


Koi Fish can live up to 25 – 35 years in captivity when conditions are right. There are records of this fish living up to 50 years in large ponds but this is not very common and most koi will live around 25 to 35 years. 

Koi are believed to live the longest among freshwater fish. In 1997, a Japanese Koi died which is believed to be the longest-living freshwater fish ever recorded. This fish was believed to live for 226 years before dying in 1977.

Behavior and Temperament

Koi fish are docile and social fish that get along with other peaceful, similar-sized fish. However, they are hungry machines that will want to constantly eat. They will consume whatever edible they can find. This can be small crustaceans, plants, or anything they can catch. 

Koi are known to be quite sociable, swimming up to humans in search of food. They will feed from your hand and will always make sure to greet you when you come close to their tank. They’re also very intelligent creatures and can identify their owners. 

These fish will show minor schooling behaviors but they are not schooling fish. Koi are social species and try to stay near the group. 

Koi Fish Care

Koi Fish are easy to take care of. However, it is important to give these fish a large living space. Koi are fast-growing fish and will need to have a large space from the beginning. They will also need a balanced diet to be able to naturally grow. 

In this section, we will teach you how to make a better living environment for your koi fish.  

Group Of Koi Fish In Pond

Tank Size

Tank size is the most important aspect of keeping koi in an aquarium. Koi fish are technically pond species and you should not try to keep them in an aquarium unless you have a gigantic tank. These fish will grow very fast so they need a large tank or pond right from the beginning.

If you decide to keep them in a tank you will have to make sure you have a 250 gallon tank or bigger. Koi need more horizontal swim space so a long tank that is 2 – 3 feet deep is ideal. The larger the tank the better. 

We, however, recommend creating an indoor pond instead of a tank. This is an ideal way of keeping koi indoors. These fish will grow big and need a large living space that lets them grow. An indoor pond is the best way to keep them happy and enjoy them while you’re inside your home. 

What To Put In A Koi Fish Pond Or Tank

Koi Fish can live in any space with almost any decorations but live plants are not recommended. These fish will eat most types of plants, so we do not recommend having live plants in their tank. However, if you want to try to have plants in your tank you can add hard-leaf types of plants. 

Plants like Anubias, Buce Plants, Cryptocoryne, Java fern, and Water lettuce seem to do fine when kept with smaller koi. Hard-leaf plants are not easy for the fish to eat so they just try to uproot them. So you will need to attach the plants securely to objects and decorations to prevent uprooting.

You might want to read our detailed guide on types of plant you can keep with koi fish.

Keep in mind that Koi Fish should have open space so they can swim freely. No sharp or rough objects should be placed in a Koi tank or pond. Sharp objects can scar the fish and as a result, your fish can get infections. 

You can add a thin layer of the sand substrate so your koi can play and search for food. Bare-bottom tanks are also another option that many fish keepers will decide to go with. In a bare-bottom koi tank, fish will easily find all the food and eat before it can rot. This can be a very good thing but the downside is that not enough beneficial bacteria can grow in a bare-bottom tank. 

Water Parameters

Koi Fish are cold-water fish and can tolerate cooler temperatures with no issues. But they can also tolerate tropical temperatures. In an indoor aquarium or pond, room temperature is the best range for these fish.

You’ll also need to cover the top of your aquarium with a hood to reduce evaporation. Water change in a larger tank is often time-consuming so by adding a hood you’ll keep the water contained in the tank for a longer time. The fish might also splash the water which your hood can stop the water loss.  

The location of the tank is also important to think about. These fish are also sensitive to noise so it’s ideal to place their tank somewhere that is not too noisy. It’s ideal to have them close to a window where they get direct sunlight. 

Koi Fish prefer slow to moderate-moving currents. Fast-moving waters will cause a lot of stress for your fish and cause them to get exhausted easily. The tank should also have very stable water parameters with no fluctuation. 

Adding salt to your tank will help your Koi Fish to stay healthy. A tablespoon of aquarium or sea salt for every 5 gallons of water is recommended. But ideally, you will need to check the salinity of the water to have a 0.1% salt water solution.

When it comes to the rest of the parameters, you can shoot to keep them in the range mentioned below:

  • Temperature: 60 – 77 Degrees Fahrenheit 
  • PH: 7.2 – 7.5
  • TDS: 50 – 150 PPM

Outdoor koi are tougher than indoor koi. They can tolerate icy cold waters in winter as long as the water is deep so it doesn’t freeze completely. If you are living in super cold regions like up north in Canada you might need to transfer your koi to an indoor pond or aquarium during the winter or add a heater to your pond. 

Koi Fish In Aquarium


Koi do not have particular lighting requirements for their aquarium. But they need to have a shaded area to hide when they are under stress. All-natural sunshine or a powerful aquarium light can be great for this fish. The light should illuminate for 8 to 12 hours a day at a specific predefined time.

These fish have evolved to survive in a wide range of environments. They will easily adapt to any lighting conditions. But an LED light that has blue and red lights embedded, can make your Koi fish look more beautiful by bringing out their true colorations. 

Filter System

Koi Fish can produce a lot of waste when fed more often. This means they need a strong filtration system that can keep the water clean at all times. A proper clean aquarium is crucial to make this fish live its full expected lifespan. 

Your pet fish’s health depends entirely on the strength and quality of the filter you are using. A good filter that can handle the bioload generated in the tank is necessary to keep your fish in a healthy environment. Most people use more than one filter to ensure water quality. 

Changing the water on a regular basis is another way to keep the water quality up. It is best to change 10 percent of the water weekly to make sure your tank parameters stay stable and clean.

Common Koi Fish Diseases & Health Issues

Koi Fish are generally hardy and will not easily develop health issues. That said, many of the illnesses these fish experience are due to low water quality and sharp objects that score their skin. In a clean aquarium that is regularly maintained and has high-quality water, these fish do not appear to contract any illnesses.

Ich, Flukes, Fin rot, Fish Lice, and Fungal infections like Cotton mouth are some of the most common diseases a Koi Fish can contract. While you might never experience any of these diseases, it is important to know these fish can get them. These diseases are often introduced to your tank by new fish that carry the disease. So always remember to quarantine all the new fish before adding them to your tank. 

Maintaining water quality is the best way to fight and prevent most aquarium diseases. By staying on top of your water changes you can ensure a cleaner tank that will not naturally develop any diseases. A good filtration system will help you prevent ammonia-related diseases in your fish. 

If your koi got any of these illnesses, you will need to seek professional vet advice on how to cure it. These fish are expensive and a misdiagnosis of a disease can cause permanent damage to your fish or even death. 

Diet and Feeding Requirements

The good news is that koi fish are extremely popular around the world which has made companies develop special foods just for koi. There are many high-quality commercial koi foods available that will satisfy the needs of this beautiful fish. 

Koi are bottom-feeding omnivores and will eat any food you give them. However, most koi foods are made to float on the surface of the water. This is mainly because fish keepers enjoy watching the fish eat while feeding them in a pond. 

However, in an aquarium environment, it is not necessary to feed them floating fish food. You can feed your fish with sinking or floating shish foods of your choice and enjoy them cleaning the bottom of the tank. 

The temperature of the water has a direct correlation to the amount this fish can eat. Warm waters will cause the fish to eat a lot more and get hungry a lot faster. But cold waters will cause the fish to eat less or completely stop eating. These fish will not eat anything during the winter when they are kept outdoors. 

You can also feed them fresh vegetables so your fish get their vitamins and minerals. Green peas, cucumber, spinach, and other fish-safe vegetables are great sources of food for your fish.

Breeding Koi Fish

Koi Fish are egg scatterers species that can breed in a home aquarium. However, it is not very often to see them breed in tanks. These fish usually breed naturally when kept outdoors. If your tank or pond has all the elements they need, it will breed without any work from your side.

In order to be able to spawn, this fish needs a place to place its eggs on. They usually lay their eggs in the shallow parts of the pond where plants are growing. In an aquarium, you will need to have a mop brush to simulate such an environment. 

The breeding process can become violent and will take a few hours. The male and female will often swim in a circle right before they want to swim. This dance can take a few minutes or a few hours before the female finally decides to lay her eggs.

If you notice that your fish have successfully set eggs and you’d want to watch those eggs hatch, you should keep the eggs in a separate container from the adults. If the eggs aren’t protected, the adult fish will consume them. 

In a single spawn, a female Koi fish may lay up to 100,000 eggs. The eggs will stick to the brush or plants and will stay there for up to 48 – 72 hours before they hatch. The adult koi will eat the eggs if you do not remove them from the tank. So if you want to have thousands of tiny koi, we recommend removing the eggs and placing them in a different tank. 

Tank Mates

Unfortunately, there are not many fish or plants that you can successfully house with these fish. These fish are omnivores and will eat any small invertebrates or fish that they can catch. However, they are not aggressive and will not actively hunt other fish. You can successfully keep these fish with other koi or Goldfish. 

Goldfish are great tank mates for koi as they both share a similar diet and will need similar water parameters. You can also keep larger species of plecos with your koi only if your tank temperature is on the higher end. Sailfin Pleco and Common plecos are some of the best plecos you can keep in with your koi. 

Koi Fish In Aquarium


Koi Fish are beautiful species of carp that are domesticated and selectively bred by humans for their coloration and meat. The current Koi Fish looks completely different from its wild type but can still live in a similar environment. These fish are mainly pond species but under special conditions that we point out in this article, you can keep them indoors in larger aquariums. 

If you still have any questions you can always reach out to us vis the contact us page. We already answered some of our readers questions in the Koi Questions & Answer article. You might find your answer there!

Japanese koi