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Will Koi Eat Goldfish?

Will Koi Eat Goldfish?

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Fish and other aquatic animals play a crucial role in your aquaponics system — they feed your plants. Koi and goldfish are two of the most popular fish types you can use for such a structure because of their aesthetic appeal and availability, respectively. However, putting them together in one environment might pose a problem. Will koi eat goldfish?

Yes, koi will eat goldfish. You should only keep these two fish species together if they are roughly the same size. These fish types will eat anything they can fit in their mouths, even smaller fish. Fancy goldfish, particularly in smaller varieties, are an attractive food choice for koi. In this article, we will explore in more detail mixing koi and goldfish.

The Origins of Koi and Goldfish

The Origins of Koi and Goldfish

Koi fish are a variety of common carp called Cyprinus carpio. that originated from Japan. This fish type is known for its aesthetic appeal because of its colors, patterns, and scaling. Today, there are over one hundred koi varieties available worldwide.

Our Chinese ancestors from over 900 years ago bred goldfish from an Asian carp called Carassius gibelio. This fish type produced color mutations that impressed the Chinese, leading to widespread breeding. Because of its growing popularity throughout the years, there are now between 200 – 400 goldfish species.

Koi fish and goldfish come from the same family, with many experts considering them cousins. Apart from being members of the minnow family, they are both docile species and omnivores. Even though they have their differences, in certain conditions, you can still raise both species in one aquarium.

Keeping Koi and Goldfish in One Fish Tank

Keeping koi and goldfish in one fish tank makes sense — they are docile species and have similar care requirements. While koi will eat goldfish in specific circumstances, it’s still possible to keep them together. With a few precautions, you can introduce these species to the same living environment.

Size Differences

As mentioned earlier, one of the most crucial aspects of putting the two species together is the sizing. You should only keep koi and goldfish together if they are about the same size. Otherwise, you risk having the smaller fish eaten by the other. Many aquaponics practitioners make the mistake of putting two-foot koi in the same tank as two-inch goldfish.

Koi and goldfish both eat almost anything organic, from plankton to vegetables and dead leaves. However, they are also opportunistic eaters and will feed on smaller fish if necessary. Koi will not just eat smaller goldfish. Without other food sources, they will eat any fish that fits in their mouths.

Environmental Aspects

In general, tanks are not suitable habitats for koi. They grow over two feet long, making it difficult to obtain a large enough aquarium for them. Even as small fish, they need a lot of space to thrive. Once they reach a length of six inches, it’s time to move them to a pond setup.

When setting up koi and goldfish together in a tank, add substrate and hides to give them space away from one another. Additionally, make sure to install water heaters and a reliable filtration system. Both are high-waste species that may not survive sudden temperature drops.

Natural Temperament

One of the best things about keeping koi and goldfish in one fish tank is their natural temperament. They both have happy personalities and will usually not provoke fights. You can raise them in a low-stress environment without much effort.

Breeding Factors

Before setting up a shared tank, consider the species’ breeding habits. Koi typically spawn twice a year, but goldfish lay eggs several times a year. Koi and goldfish can also interbreed to produce a unique species.

An overpopulated aquarium may pose a threat to your aquaponics system because koi might feed on eggs and smaller fish. This fish type will feed on its own eggs if necessary.

Top Aquaponics Choices

Aquaponics is a groundbreaking farming method that uses fish waste as fertilizer for plants. The plants remove toxic compounds from the water, providing a safe recirculating environment for the fish.

Koi and goldfish are two of the top choices for aquaponics settings if you consider maintenance difficulty and temperament.


Koi fish may thrive in aquaponics settings under ideal circumstances and will add a burst of color to your tank. This fish type typically lives a long life because of its resistance to parasites and ability to withstand a wide pH range.

Because of its massive adult size, you will need at least a 200-gallon tank to house a school of this fish. However, you can transform koi ponds into efficient aquaponics systems.


This fish type is ideal for in-home aquaponics systems because of its availability, affordability, and waste production. If you want to start an ecosystem immediately, goldfish is the best species for you. It’s a docile breed, and you can conveniently raise a school of them with similar-sized fish.

Conclusion: Will Koi Eat Goldfish?

Yes, koi will eat goldfish. However, if you set up the ideal aquaponics environment, you can successfully raise koi and goldfish together. Consider the following before starting an ecosystem with these species: size differences, environmental aspects, and breeding factors. With research and dedication, you can build an aquaponics system that includes both these beautiful breeds.

The Campbells love finding sustainable and fun ways to increase their independence from traditional brick and motor supermarkets. Aquaponics provides a full lifecycle food source for families and a great hobby. #aquaponicslifestyle

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