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Can I Keep Koi and Goldfish In the Same Tank? All To Know

There is a simple answer to this question. Yes, You can keep Goldfish (Carassius auratus) and koi together in the same tank, but there are some problems with that arrangement. Goldfish are very useful breeders and breed more than once a year. Due to that reason, there will be more goldfish in the tank than koi fish. High fish numbers in the tank easily change pond water conditions. Among koi and goldfish, goldfish can tolerate hardy environmental conditions than koi fish. The result is clear to you now. The union of these fish will result in deadly consequences for koi fish.

Algae eating fish for ponds

Other than that, keeping these tropical fish together can interbreed and will produce useless offspring. Koi fish are a result of a selective breeding process, and Koi fish breeders had spent a lot of time and effort to create a good Koi variety. Therefore it is not cost-effective to let these fish interbreed. also if you are keeping them together bellow details will help you

Can I Keep Koi and Goldfish In the Same Tank All To Know infographic

What Goldfish And Koi Have A Lot In Common?

Both Koi fish and goldfish come from a common ancestor, and they have some common features.


Both Koi and Goldfish are omnivores, meaning they eat both plants and meats. They like to eat small crustaceans, various plants, fish, eggs, and worms. Another feature they have in common is pharyngeal teeth, and they use it to grind up their food. These two types of fish lack stomachs.

The food they ingest directly goes from mouth to intestine. In the intestine, all the essential nutrients absorb into the body. However, Koi fish have a much larger mouth than goldfish and use their bigger mouth to dig deep into the mud to search for food. especially, a koi fish’s mouth is large enough to swallow small fish and you should consider this when introducing very smaller fish into Koi pond.

-Water quality

As they are tropical fish, Koi and goldfish require more or less similar temperature, pH, hardness, and nutrient range in their water. The optimum temperature for these fish lies between 65 and 75 F degrees. Both these fish can tolerate sudden temperature changes.

When considering the pH, Koi, as same as Goldfish like 7.4 – 8.4 pH range. They can live in waters with a wide range of pH, but they can not tolerate sudden pH changes in the water. Recommended pH variation is 0.2 / hour. As you may know, pH can explain the acidity of the water. This acidity can be changed due to various reasons.

In Koi and Goldfish aquariums pH changes could happen due to plant photosynthesis and fish respiration. The best hardness range is 150-300 mg/liter. In the aquarium, water hardness can fluctuate quickly depending on the bottom substrate.

In a native environment, both fish (especially koi) prefer mud bottom—this mud bottom leeches carbonate ions into the water, which influences water hardness.


Both Koi fish and Goldfish need a lot of space to move around. Most fish lovers prefer to put goldfish in a bowl. But that is not comfortable for the fish. Therefore choosing the best space for both fish types are very important. Koi fish especially need an outdoor pond to swim around freely. They need room to explore and nibble around.

Interestingly koi is an eye-catching sight in the pond than in a glass tank. You may think goldfish prefer glass thank than a pond. But goldfish also prefer ponds rather than glass tanks. However, the beautiful fins and other goldfish body colors make them the better choice for a glass tank.

Planning to keep them together spacious outdoor pond is a wise choice than an indoor tank.

-Habitat Maintenance

With their Asian wild heritage, Koi and Goldfish like to live in a pond with lots of plants. Koi fish want to eat plant materials in the pond and dig the root and uproot the plant. Like Koi, Goldfish try to uproot the plant from the substrate, but they do not like to eat the same plant as Koi fish.

When considering the bottom of the pond, koi fish prefer mud as the bottom substrate. Goldfish do not have such a preference, but they can live in mud bottom ponds.

Pond or tank maintenance is the same for both these fish. At least once a month tank should be cleaned. Overgrown and rotten plants must be appropriately treated by removing the overgrown/rotten plant material.

Other than that, cleaning the skimmer and filter, removing deposited sediments, and partial water changes (up to 10 to 25 %) should do at least once a month. As the quality of the water is important for these fish, make sure to conduct regular water checks.

The Difference Between Koi And Goldfish


Although some people have trouble differentiating Koi fish and Goldfish, they are totally different in size and body shape. Goldfish are generally fat and egg-shaped, while koi carp are elongated and torpedo shape. Both fish types have different types of fins.

Goldfish have a separated tail fin as well as the dorsal fin. Unlike goldfish, koi have connected tail fins. The most notable difference in appearance is their body size. Mature Koi can grow up to 4 feet, while goldfish can grow up to 1 foot long if enough space is given. But most of the time, koi fish have grown up to 25 -28 inches and, goldfish have grown up to 4 inches.

Koi and goldfish have different color combinations due to years of selective breeding. Specially koi fish have different color verities based on white, orange, black, and blue. Koi carp breeders have named over 25 unique realities depending on their body color patterns. For example, Goshiki is a Koi variety with red, white, brown, and blue accents. Goldfish, on the other hand, do not have as complex color combinations as Koi fish. Their skin color is mainly produced by black, red, and yellow pigments.

Goldfish with no pigments are white in color. They live much longer than most other aquarium species. The oldest known koi fish lived an impressive 226 years from 1751 to 1977 in Japan.

-Growth and Lifespan

A healthy koi fish can live up to 50 years. Although goldfish can not live as much as koi, they also can live up to 15 years or more in the well-maintained aquarium. Longest lived goldfish was 43 years old when it died.

Pros and Cons of Keeping Koi and Goldfish Separated/Together

  • Although we can keep both fish in the same pond, there are more cons than pros.
  • The only pro I can think of is to save space. You can easily keep these two fish types in one outdoor tank rather than building two separate tanks. Apart from that, they are pretty to look at in an outdoor pond.
  • Here are the cons. As you may know, goldfish reproduce like a rabbit (or rats). They can lay eggs more than twice a year. If this happens in a Koi and Goldfish pond, the pond will be overcrowded sooner than you expect. And it is a very hard job to remove all the goldfish once you are introduced into the pond. If you missed some goldfish once you are cleaning the pond, again, there will be more goldfish in the pond after two or three months.
  • Apart from that, Goldfish will mate with Koi and produce unattractive baby fish. These fish have no aesthetic or commercial value. Also, they grow much faster than other fries in the pond.
  • Moreover, koi need more water and space to thrive. If space is packed with goldfish and their newborn babies, koi fish can not grow properly in the pond. Besides that, if there is any disease outbreak, these two types of fish should be treated differently. It is not possible if both fish in the same tank.
  • Most importantly, people manage to keep both fish in the same tank for 2 – 3 years. But they also prefer to keep Koi and Goldfish in different tanks due to the above-mentioned reasons.

Will Koi kill Goldfish?

Goldfish and koi fish are not aggressive toward each other in normal conditions. They live peacefully in an enough-spaced pond. But if you kept them in a small pond or a tank, there will be problems.

Koi fish need a lot of space, and if someone tries to keep Koi and Gold in a small space, there will be rapid water quality changes, which affect Goldfish or Koi fish. Other than that, it is not wise to introduce smaller goldfish with mature Koi fish. As koi carp are omnivores, they might try to eat smaller goldfish.

This only happens if there is not much space to move in your pond or tank.

Why The Goldfish Chasing My Koi?

This is not anything unnatural. Goldfish usually do not chase Koi as Koi fish are much bigger than goldfish. But sometimes, Mature male Goldfish are mistaken for female koi fish for female goldfish. When natural mating instincts arise, goldfish try to stimulate a female animal’s egg release by chasing around the female.

This chasing can get intense since female koi fish will not be ready to release eggs. If it looks like the female koi fish is tired, the first thing you should do is to remove the tank’s male goldfish. Otherwise, it could be fatal.

Can Goldfish Survive In a Koi Pond?

Yes indeed. Goldfish and koi fish can live peacefully in the same pond. The most important fact is, that the pond should have enough space for both Koi and Gold. But it would help if you were careful when introducing tiny goldfish into a pond with mature Koi fish. As koi fish are omnivorous, they may see tiny goldfish as food.

It is better to introduce developed goldfish into an already established Koi pond and make sure the pond is large enough for them to swim.

Can Baby Koi & Adult Goldfish Live Together

Introducing very tiny (less than 1 inch long) koi fish/ koi fry into a goldfish pond is not the right choice. As goldfish are eager to eat anything that can fit into their mouth, tiny koi fish could be an easy meal for mature goldfish. Therefore, keep your Koi fish separate, until they reach 1-inch body length. Other than that, goldfish are not aggressive toward baby Koi fish.

Frequently Asked Questions

-Can Koi Fertilized Goldfish Eggs

Yes, Koi and Goldfish can produce offspring together. But these offspring cannot produce eggs of their own. These hybrid’s skin appearance depends on their parent’s skin.

-Why Do Koi Fish Eat Goldfish?

Koi fish are omnivores and eat anything that goes into their mouth. That also includes smaller fish. If the Koi fish is hungry enough, they will try to attack small goldfish and eat them. In a normal situation, koi do not attack goldfish. providing large

-DO Koi Fish Eat Goldfish?

Yes, if the goldfish is smaller than the Koi fish’s mouth and the koi fish is hungry, that can be happen

-Can Koi And Goldfish Eat The Same Food

Yes, Generally, both fish can eat the same kind of food. They are both omnivores and eat the plant, small crustaceans, worms, and fish eggs. In aquariums, they are fed with flake and small pelleted foods.

-How Often Do You Feed Koi And Goldfish?

You can feed Koi and Goldfish two or three times a day.

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About Dr.Chamika

Hello, I'm Dr. Chamika. I am a Researcher in Water quality, Aquatic organisms, and Environmental chemistry. I am a passionate fish keeper, with10 years of experience. My mission is to help other aquarists experience the joy of fish keeping.