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Koi vs Goldfish – What’s The Difference? And Can You Keep Them Together?

Koi and goldfish are two of the most popular species for fish lovers all over the world.

Although they are similar in certain respects, there are enough major differences between these two species that you should recognize, especially if you plan on keeping one or both of these species.

Your beloved fish deserve the best care and attention. This means they require an adequately sized pond, proper, nutritious feeding, compatible pond mates and an overall healthy, beneficial environment.

You can only provide these things if you understand what type of fish you have, and what the requirements and optimum ranges are for that fish.

That’s where we come in!

In this comprehensive overview of the great koi vs goldfish debate, we will introduce you to the similarities and differences of these two types of ornamental fish, focusing heavily on appearance, temperament, diet and water conditions.

We will also go one step further and look at which of these two fish species is more suitable for you, and your aquatic set up.

So, if you’ve always wanted to know a bit more about koi fish and goldfish, then stay reading!


Both koi and goldfish have very similar origins in the wild. Both species are originally native to the far east.

But goldfish have been domesticated for millennia now, and generations of selective breeding have meant that they have developed very different attributes from their cousins in the wild.

First of all, the common goldfish has been developed by selective breeding of Prussian carp to get special color variations. This is why they have such vivid coloring compared to their ancestors in the river.

However, the goldfish has now departed so far from their carp beginnings that it is now qualified as a separate species.

Things are somewhat different for the common koi. The koi was also bred from carp fish, where selective breeding led to the impressive size and appearance of the attractive pond fish today. However, koi are not considered to be a separate species from the carp in the wild.

Koi are also a much newer type of fish than goldfish. Then have only been bred since the 1820s and were first imported into Europe at the end of the nineteenth century.


Although goldfish and koi are distant relatives, there are dramatic differences in the appearance of both of these species.

Both fish can be very beautiful and appealing in the right settings, but you should not underestimate the physical differences between the two.

The most telling physical differences between these two fish is in terms of body shape and size.

Body Shape

There are so many distinct types of goldfish available and all of them will have subtle differences in terms of body shape.

But in general goldfish have a wide, egg-shaped body. Contrast this with the body of a koi, which is almost always lean, streamlined and torpedo shaped. This narrow body gives it an advantage in terms of speed and agility.

Another visual difference comes in terms of fin structure. The dorsal and tail fin of the goldfish is usually split, while the fins of a koi are exclusively connected.

To distinguish koi vs goldfish  you can also look for barbels.

These will appear like whiskers near the mouth of the fish, and will only be found on koi and not on goldfish.


Size might be the most apparent difference to the casual observer. Goldfish tend to be much smaller than their koi counterparts, which continues when they reach full maturity.

It is not uncommon for Koi to grow to be over 20 inches long. On the other hand, when goldfish reach adulthood, they are generally less than four inches in length.


There is a lot less difference in terms of coloring. Both species have been so heavily domesticated over the years that a great variety of different coloring have developed. Both will tend to display their coloring more vividly in situations where they are happy, healthy and not stressed.

You can find red, orange, black and yellow types of both koi and goldfish. However, there is generally more color variation to be found in the world of koi fish than in the world of goldfish.

Koi can display metallic type coloring as well as the duller shades. These dramatic combinations can make a group of koi in a pond a really aesthetically pleasing feature.

Lifespan and Hardiness

When considering whether to invest in goldfish or koi, this is one of the most important considerations for any aspiring pond owner. You need to know how long you can expect your fish to live, and whether it will be worth it.

Hardiness refers to how resilient the fish are, and how tolerant they are of water conditions changing somewhat.

You want a hardy fish so that a brief transient in water conditions doesn’t kill off all your stock.


Video: “The World’s Oldest Koi – Hanako (226 Years Old)”

There is a clear winner in the koi vs goldfish argument when it comes to lifespan. You can expect koi fish to live twice as long as goldfish, as a general rule.

For example, the average lifespan of a goldfish is between 5 and 10 years, but the koi can greatly outmatch that.

The average lifespan for a koi fish in captivity is between 25 and 35 years, which is really not bad innings!

However, some fish have been known to greatly outlast even that. For example, the oldest living koi fish in the world is an incredible 226 years old!

Both koi and goldfish tend to reach full maturity in a similar length of time, between 2 and 3 years. Before that time, they are smaller and less well developed, so they are most vulnerable and need to be protected from larger or more aggressive fish.

If you want to get more bang for your buck in terms of lifespan, then it is clear that koi fish are the way to go.


In terms of hardiness, both species have their advantages. For example, the common koi is reasonably resilient to many different types of parasites and infections that could prove fatal to lesser fish. This may derive from their ancestral origins or not.

However, you should be aware of the koi herpes virus, which is a great threat to koi fish. They will also not do very well is there is a sudden, dramatic shift in water conditions such as pH or hardness. This could greatly damage the ecosystem and the overall health of your koi, so your pond should be kept clean, aerated, well filtered, and stable.

Goldfish are also a reasonably tough little creature, but you should keep a close eye on them for signs of illness, injury or infection.

Fancy types may be more prone to illness, and less resilient of colder temperatures, than common goldfish.

Ideal Conditions

Koi are well known as an outdoor fish, as koi ponds have become a more and more popular feature in many buildings worldwide. However, although it is not as common, many breeds of goldfish can also be bred in outdoor ponds, if special precautions are taken.

Koi fish can be kept outdoors all year round, which is a blessing because they are probably too large to be transferred indoors.

They can adjust to seasonal changes in weather because of a special trick that allows them to lower their metabolism to basically nothing during the winter. They essentially hibernate and do not need to be fed during this period. They are most active in temperatures above 20 C.

Goldfish are commonly known as a cold-water fish, and some fish can survive freezing winter conditions. However fancy variants will not survive such low temperatures.

It is best practice to transfer all goldfish inside for the winter. The ideal temperature for goldfish is 68 – 72 F, and the ideal pH conditions for both species is just above neutral, from 7-8 pH.


Video: “When do I start to feed my pond fish – feeding my fish”

The dietary requirements of both goldfish and koi are very similar. You should choose a fish food (either in pellet, tablet or flake form) which fulfills all their requirements in terms of protein, energy, vitamins and minerals.

Both types of fish are opportunistic feeders who will continue to eat even after they’ve had enough, so you need to limit how much they eat sometimes. Otherwise your pond may become dirty and algae filled due to an overabundance of waste food.

The most notable difference between koi vs goldfish is the relative size, so koi will need to be fed more.

Do this regularly, about three times a day.


The great koi vs goldfish debate has raged for a long time. Hopefully this article helps to expose some of the similarities and differences between these two breeds, and should help push you towards whichever breed is more suitable for you.

If you want a larger, more long-lived species then you should consider opting for the koi. However, for beginners, with limited space or filtration capacity available, the goldfish may prove to be a better choice.

The origin of both species is similar, but the appearances can vary dramatically. Their ideal living conditions in terms of water quality, pH etc. are similar, but in general koi are a bit more resilient to lower temperatures, and can spend the winter outside.

Aaron Boyd
Aaron Boyd

Hello, I’m Aaron Boyd, the proud owner and author behind Aqua Movement. I hope my article was able to answer your questions. If you want to learn more about me, click the home icon above.

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