Koi are a beautiful type of decorative fish that are becoming more and more common in home aquariums. When lined up next to the average carp, they can look quite the same to those unfamiliar with the species.

But did you know that the koi fish actually descended from the carp? Not only that, but this is a fairly recent development, too. It is also worth pointing out that koi fish are not carp, despite their somewhat similar appearances and traits.

Here is a comprehensive guide as to the differences between the carp and koi.

Body Shape

One of the main things that help to tell the carp apart from koi is their body shape. The fact of the matter is that the two are quite different in terms of their overall shape. The carp, for example, has a body that is a bit more compressed and taller (not longer) than the average koi. A lot of the carp’s growth is in their midsection while the koi fish will usually grow a bit longer.

Through selective breeding, the koi fish has developed an almost torpedo-like body shape where they have a narrow head and tail with a thicker midsection. This is the biggest difference between carp and koi fish.

Coloring

As much as their body shape makes them different, perhaps the most important and obvious difference between the carp and koi fish is in their coloring. Carp tend to be a more neutral, darker color. They tend to be a golden or dark brown color that begins to fade into a lighter tone toward their belly and fins.

Koi fish, meanwhile, are vibrant and bright, coming in a wide range of patterns and colors. You have no doubt seen koi fish in yellow, red, orange, blue, black, and white, among other colors. This is part of the reason why they are such a popular addition to so many home aquariums.

Their vibrant colors make them a wonderful decorative addition, becoming a standout addition to any home aquarium. It is also partially the reason why they tend to be quite a bit more expensive than their carp counterparts.

Fins

One of the most unique differences between the common carp and the koi fish is in their fins. To the untrained eye, it may not be the first thing that jumps out about the two, but it can be as telltale a sign as anything else.

The carp has an elongated dorsal fin while also having a separated spine in each of their fins. This isn’t necessarily noticeable, but it is a major difference from the koi who lose those spines. This is what allows their fins to look almost majestic and flowing, particularly in the butterfly koi.

The carp tends to look a little more “basic” in every aspect whereas the koi is more vibrant, flowing, and unique.

Size

Though the koi fish are so decorative and beautiful, they actually can be quite a bit bigger than their carp counterparts. The koi fish can grow to be as large as 24-inches by 28-inches in length. This is in contrast to the carp, which can get to be about 15-inches to 27-inches long.

The carp can actually grow to be quite large, though, in terms of their weight. Carp can get to be pretty large with the biggest carp ever weighing in at 105 pounds. “Big Girl,” the largest koi on record, weighs in at just 91 pounds.

The difference isn’t substantial, and it is possible for the carp to have a larger appearance despite measuring in at a smaller length.

Scales

The last large difference between the two fish is in the scales. Both koi and carp lack scales entirely – these are known as leather carp or leather koi. When they do have their scales, the koi fish’s scales are more curved, regular, and much smaller than that of the traditional carp.

Their scales are also much brighter and more attractive than the traditional carp. This is why it is not uncommon to see koi in tanks and aquariums in a decorative setting.

Koi are also naturally susceptible to predators. With their beautiful, vibrant colors, it is difficult for them to avoid their prey. Their fins, while beautiful and flowing, also make them substantially weaker swimmers when compared to the carp.

Additionally, their curved, smaller scales don’t give them quite the layer of protection that the carp get from theirs. It is partially why they are so abundantly available in a decorative manner.

The last huge difference between them is the habitats that they permeate. Carp are a hardier type of fish that can be found just about anywhere in the world. Koi tend to do best in ponds that have been carefully crafted to display them rather than out in the open water (remember, they are weak swimmers and have no camouflage).

Buying Koi Fish

While carp isn’t a particularly common fish to find in tanks and ponds around the world, the koi fish definitely is. Because of their bright coloring and majestic fins, koi are almost a trophy fish more than anything else.

If you’ve ever shopped around for koi fish, you know that they can get to be pretty pricey depending on what you’re looking for. For instance, back at the All Japan Koi Show 2017, the show’s grand champion was sold for $1.8 million, making it the most expensive koi fish of all-time.

But why are koi fish so expensive? The main reason is because of the prevalence of koi breeding. Koi breeding began all the way back in the 1700s in Japan by rice farmers in the country. Similar to the way that dogs are bred for their individual traits, koi fish were selected for their specific patterns and colors.

Collecting koi fish is a very serious hobby in some areas and both collectors and breeders take the entire process quite seriously as well. The major koi fish shows are an indication of this, as is the rather large price tag of $1.8 million for a single koi fish.

So, why are koi fish so expensive? It turns out that they have a few traits that make them highly collectable and quite costly.

Bloodlines

To get the most brightly colored, wonderfully aesthetic koi, Japanese breeders began looking at the bloodlines of the koi. One of the koi’s ancestors, the wild carp, have a darker color that is considered to be less attractive for koi despite the fact that it helps them survive better in the wild.

A prize koi will tend to exhibit the exact opposite of those genes. Instead of muted colors, better for survival, they have colors that are wild, vibrant, and as attractive as it gets. The more their colors stand out, the higher the price they are likely to fetch.

Koi will have six different types of color cells in their skin: yellow, red, white, black, blue, or a metallic sheen. The most prize of kois will come from bloodlines that have a lot of red cells burgeoning in big patches with bodies that are white.

In Japanese culture in particular, those white and red colors are very important. Not only are they the colors of the Japanese flag, but they represent purity and joy as well. This is a major reason why white and red koi fish are so highly sought after in terms of their breeding potential.

Special Traits

Prize koi have certain standards that have to be met among collectors and hobbyists. They have to be of a certain body shape, size, and health standard, including the way in which they move through the water.

But the biggest quality in koi fish is how they are able to stand out from an aesthetic standpoint. The best koi will have a nice balance of patterns and coloring, though this can depend on their variety as well.

The three colors that the majority of koi enthusiasts will look for is koi with larger red patches on their backs, lighter koi with blue spots, and koi that appear to be completely metallic gold.

While colors and aesthetics are so important, it is possible that there are some colors that just won’t look very good. The fish’s cells that hold their color will sit at different depths in their skin. Some of them will sit deep under the skin while others will rest right at the surface. These factors can greatly impact the brightness and aesthetic appeal of the koi fish in question.

Breeding

Did you know that as of today, there are about 120 total varieties of koi fish around the globe? Not only that, but a single koi can give birth to several hundred thousand baby koi in a single birthing session. So, to get the best koi, the breeder may find themselves with several million baby koi on their hands.

The breeders will tend to make a few rounds of selections based on their criteria as these fish continue to grow. The cuts can be drastic: they may go from as many as three million babies down to about 15,000 to 20,000 or so during the first year, then narrow it down to about a thousand the following year.

Koi fish can keep growing for as long as nine years – that was how long the aforementioned grand champion koi grew before being sold. Needless to say, breeders are making a huge investment into their setups and inventory of koi.

What Is the Average Cost of a Koi?

Generally speaking, when it comes to koi that are pond-quality, the price range can depend on a number of things. You can find them for as inexpensive as $10 or $100 on a regular basis. It really depends on the size of the koi fish in question.

But show-quality fish can be the real bank breaker. The best koi in the world for show have been bred for just that purpose. This also means that their bloodlines are relatively stable, somewhat guaranteeing what their primary color traits and characteristics will be.

For serious koi collectors, spending thousands on an individual koi is all too common. Most serious koi investors will often breed their own koi fish, hoping to create the kind of prospect that can reach grand champion status.

Which Koi Are the Most Expensive?

As talked about in previous sections, the red and white fish are very sentimental and important to Japanese collectors. But there are more collectors in the world than the Japanese, so what are some of the other color types that bring the largest price tags?

Koi that have light blue spots and others that are all-metallic gold tend to be quite popular (and expensive) as well. These three color variations are often the most valuable and tend to dominate the show circuit, winning just about every major category in these shows on a regular basis.

Can You Tell How Old a Koi Fish Is?

When the koi fish begin to mature, it can be quite a bit easier to tell them apart as far as their ages. Generally speaking, the biggest factor in determining the age of a koi fish is in their body length. When they get to be 10 or so inches long, they are generally three years old and are then considered to be mature.

Any shorter than the aforementioned 10 inches and the koi fish is considered to be in the juvenile category. Most of the champion koi fish in the world will grow for seven to nine years before they are sold, making them much larger and more mature than their younger counterparts.

Koi are quite beautiful and vibrant, making them great for collecting. They swim gracefully throughout ponds and even some larger home aquariums. But be ready to pay a pretty penny if you want a competitive koi fish. The koi fish competition is a tight one and only the strongest (and wealthiest) will survive.

Author

I have a bachelor's degree in construction engineering. When I’m not constructing or remodeling X-Ray Rooms, Cardiovascular Labs, and Pharmacies...I’m at home with my wife, two daughters and a dog. Outside of family, I love grilling and barbequing on my Big Green Egg and working on projects around the house.

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