How Big Do Koi Fish Get? Growth Rate, Tank Conditions, & More (2024)

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Koi were originally wild-caught by rice farmers in Japan and bred selectively to create the modern koi fish we know today. Koi fish are actually just stunningly colored varieties of Amur Carp. If you want to keep this dragon-like fish in an indoor aquarium, you’re probably wondering how big do koi fish get.

On average, koi fish can grow up to measure 36 inches in length when fully matured. They have a growth rate of 0.8 inches per month, depending on the variety of koi you own.

These ornamental carp varieties typically have the same build, but by identifying their distinct colorations and patterns, we’re able to classify over 100 different koi fish varieties.

While their color patterns fascinate most fishkeepers, it’s their impressive size that puts them front and center in any aquarium or pond that they inhabit.

This article will walk you through everything you need to know about the growth rate, tank requirements, and water conditions that come with attempting to grow your own dragon koi fish in an aquarium!

So, How Big Do Koi Fish Get?

On average, domestic koi fish varieties living in an aquarium will grow to be about 12-15 inches long, while traditional types of Japanese koi will grow up to 26 inches in length. Jumbo koi varieties, which were bred selectively to maximize their body mass, can reach a length of 36 inches, weighing between 22 and 26 pounds.

Each type of koi fish (from over 100 varieties!) fits into a slightly different size category, and that’s because their size as mature individuals is usually linked more to their genetic makeup than it is to the environment they’re living in.

One popular myth about koi fish is that they will only grow as large as their aquarium (or pond) allows them to grow. This myth doesn’t stand on any solid arguments but stems from the fact that, in a way, tank size does influence a koi fish’s growth.

Similarly, the diet a koi fish will be fed throughout its lifetime, water quality and temperature, are all factors that will have a strong impact on a koi’s growth.

What the myth fails to acknowledge is that tank size & co will actually determine a koi fish’s growth rate, not how big they will get.

You can take control of the external factors that determine how fast they grow and give your koi the best possible care to reach its full potential size quicker. But it will not grow larger than its genetic-based size standard!

How Long Does It Take A Koi Fish To Grow To Its Mature Size?

Most koi fish varieties will reach their mature size in the first three years of their lives.

That’s typically considered a fast growth rate, considering the fact that some types of ornamental carps can live well into their 60s (Japanese koi in particular).

Domestic koi fish will reach a length of 9 inches by the time it celebrates its first birthday.

Western bred varieties tend to have an even faster growth rate, but that comes at the expense of a shorter lifespan, in contrast to the Japanese-bred koi.

You can check out this Koi Growth Chart for a visual perspective on how long it takes these fish to grow to their maximum size.

Factors That Influence A Koi Fish’s Growth Rate

Koi fish typically grow at the same rate during their first year. Following this milestone, their growth rate will be determined by the quality of their diet, the water parameters inside their tank, and more.

These are the main factors that can influence a koi fish’s growth rate:

Water temperature

Koi fish kept in aquariums will grow a lot quicker than if they were to be kept in a pond, regardless of which type of koi we’re talking about.

This exponential growth pattern has a lot to do with a slightly higher water temperature. In an aquarium, there are also fewer temperature fluctuations.

A stable water temperature that ranges between 75-80°F is optimal for a faster koi fish metabolism, which results in accelerated growth.

This is one of the main reasons why western bred & raised koi varieties tend to grow quicker than Japanese-sourced koi. In Japan, breeders will not warm up ponds during the winter, encouraging their fish to hibernate under a layer of ice and slow down their metabolism to adjust.

How a koi owner manages water temperature inside a tank or pond will have a huge influence on a koi’s growth rate, but a speedier growth might come at the cost of a shorter overall lifespan.

Stress

Koi fish are typically considered hardy freshwater fish that can withstand most environmental stressors with a great deal of resilience. But they are still vulnerable to the consequences of excessive stress.

How do aquarium/pond fish get stressed? Frequent dips in water quality, water temperature that’s out of their comfort zone, incompatible tank mates, overcrowding, and poor oxygenation are just a few examples of stress factors that can slow down a koi fish’s growth rate.

Koi growth can even temporarily stagnate if your fish are overcome by KHV infections, parasite infestations, or diseases.

Food

Omnivores by nature, koi fish will eat anything and everything. They are also greedy eaters, which is one of the main reasons why they are so easy to train for hand feeding. They’ll do anything to be the first in line to be fed!

Even though they are food-driven, some dietary rules come in handy if you want to accelerate a koi’s growth rate without risking overfeeding and the health issues that come with it.

A protein-rich diet, consisting of fish food that is packed with vitamins, minerals, and necessary nutrients, is preferred. As with all fish in captivity, offering a variety of protein, carb, and fat sources during feedings will maximize their growth rate.

Even though they will constantly munch on algae and vacuum the bottom of a tank/pond looking for extra-food, what you feed them will have a bigger influence on their growth rate than what they manage to forage for themselves.

Not feeding koi enough food, or even just keeping them on a mono-diet (same food for every feeding) will significantly slow down their growth rate.

Overcrowding

Overcrowding koi fish in an aquarium will not only stunt their growth but might actually cause you to lose a few fish in the process. Keeping too many koi in an aquarium will kill most of them before their first birthday.

Koi fish are very messy and will overwhelm the culture of good bacteria in an aquarium if they’re overcrowded. No filter is more efficient at cleaning tank water and maintaining biological balance than nitrifying (good) bacteria.

If good bacteria can’t work fast enough to maintain good water quality, the water inside your koi fish tank will turn toxic (ammonia poisoning!). The same can happen in a pond that’s overstocked with koi.

The lack of space to swim and be active also has a negative impact on koi fish growth because it’s a stress-inducing circ*mstance. So, if you want to keep a happy fast-growing group of koi in your home, make sure you can accommodate their tank size/ pond size needs.

Genetics

Just like genetics are the deciding factor on how big koi fish get when mature, the genetic makeup of a koi will have a strong influence on how fast it can grow.

Western koi breeders have done their best at selectively breeding koi to create the rapidly growing modern domestic koi we know today, with some domestic varieties reaching their adult size at the age of 2.

All influential factors considered; a koi fish can’t grow quicker than its genes allow it to. Koi typically have the most accelerated growth rate while they are juveniles. This 1-year growth-spurt is genetically predetermined.

You, as a koi fish owner, will have the most influence over their growth rate during their adolescence, when they can quickly double or triple in size. As they age, the overall growth of a koi fish will halt.

There are exceptions to this rule, of course. An individual bred from strong quality parents has better chances at growing exponentially in its first two years of life than an individual with smaller-sized slower-growing parents.

Ideal Tank & Pond Conditions

To keep stress levels low and avoid unintentionally slowing down your koi fish’s growth rate, make sure you optimize these basic tank conditions:

Water quality and water parameters

Temperature

With a strong connection to how fast (or slow) a koi’s metabolism is, water temperature inside your koi fish tank will have a big impact on their growth.

In the wild, carp can live in temperatures ranging between 35-85°F, but if you want to give your koi fish the optimal water temperature for growth, you need a tank heater to maintain a stable water temperature ranging between 65-75°F.

Sudden drops (or spikes) in water temperature can be big stress-factors that can put their growth on hold, even as juveniles.

Tank size guidelines for keeping koi fish

Determining the ideal capacity that your koi fish tank (or pond) should be able to hold is a bit tricky.

The perfect tank size will depend on the variety of koi you own (domestic, Japanese, jumbo-sized, etc.), their age, whether they’re long-finned or short-finned, etc.

You can use the GPK (gallons per koi) scale to get an idea of what tank size would give your koi the best environment to grow into its maximum size:

• Small Koi (2″ – 8″): 100 – 150 gallons;
• Medium Koi (8″ – 14″): 250 – 300 gallons;
• Large Koi (14″ – 24″): 400 – 500 gallons;
• Jumbo Koi (24″ – 36″): 750 – 900 gallons;

Check out this pond volume capacity calculator if you’re planning on keeping koi in a pond.

Tank mates

Koi fish are peaceful and have a non-confrontational temperament that makes them easily compatible with most other peaceful fish. They won’t harass or eat other fish, but they can be an intimidating and stress-inducing tank mate for small nervous fish.

The most popular tank mate choice for koi, whether they’re kept in tanks or ponds, is comet goldfish. They’re just as hardy as koi fish, and they share the same preferences when it comes to water temperature and tank conditions.

In their natural habitat, carps (koi are just ornamental carp!) share lakes and streaming bodies of water with game fish, white cloud mountain minnows, and killifish.

Koi owners tend to avoid keeping them with other freshwater species that they might have to compete for food with. Bluegills, in particular, will give koi fish a hard time.

Less food to eat, less overall growth!

Biggest Koi Fish Ever Recorded

The biggest koi to date remains the “Big Girl,” a Japan-sourced koi fish owned by Geoff Lawton, owner of the Rainbow Koi shop in Wiltshire, England.

Big Girl has grown to an impressive size of 48 inches, weighing in at 90lbs.

Even before being imported by its owner, the Big Girl was the largest koi to leave the Japanese specialty breeder’s collection of jumbo-sized koi.

To maintain her weight, Big Girl is fed approximately 1 pound of top-quality koi fish food per day and is kept in her own show pool, inside her owner’s fish shop.

Holding the record of being the biggest koi fish ever recorded, it’s no wonder her owner has no intention to sell Big Girl. When questioned what offer he would be willing to accept from a buyer, he said $40,000 would be the ballpark number he would be willing to consider.

How Big Do The Most Popular Koi Fish Varieties Get?

The most popular types of koi fish are the fast-growing western-bred domestic koi, which are inexpensive to purchase as juveniles, the butterfly koi with its dragon-like figure, and the high in demand Japanese koi.

Domestic Koi Fish

With a genetic makeup that predetermines one of the quickest growth rates among all koi varieties, the domestic koi will grow to an average length of 12 to 15 inches.

Their smaller size makes them low-maintenance koi fish options for fishkeepers that don’t have space to set-up a pond and prefer to keep their koi in an aquarium.

The diverse color patterns these koi fish come with make them a stunning addition to any aquarist’s collection, without any huge upfront expense. Domestic koi are much less expensive than Japan-imported koi fish.

Japanese Koi

Traditional Japanese koi varieties grow bigger than most domestic koi, reaching lengths between 22 and 26 inches. These koi fish will also outlive domestic varieties, living 25 to 35 years on average.

Their impressive longevity makes Japanese koi a pet fish that will require a commitment to keeping it healthy and well-provided for, even if it sometimes outlives its original owner.

Japanese koi will rarely do well in an indoor aquarium, most being accustomed to large open swimming areas and a pond that’s at least 3 feet deep, ideal for hibernation throughout winter months.

Butterfly Koi Fish

Butterfly koi, also known as dragon carps, will grow to an average length of 24 inches, with some individuals topping off at 36 inches.

They can seem bigger than they actually are because of their long-flowing fins. They’re known as longfin koi for a reason! Their fins grow significantly slower than they do, so a koi’s fins might get more intricate and remarkable as they grow older.

A butterfly koi’s fins will grow as long as the blood vessels that supply them with blood will grow.

Their splendid yet delicate fins make them susceptible to fin rot. But if treated in time, butterfly koi can actually regrow their fins and tails.

Butterfly koi owners will typically keep them as a single species in an aquarium or pond or will at least avoid keeping them with fin nippers.

Jumbo Koi Fish

Jumbo koi varieties can often reach lengths of 34 to 36 inches (or 48 inches in the case of Big Girl).

The average weight of jumbo-sized koi can be anywhere between 22 and 26 pounds.

They require specialty fish food to reach these enormous sizes, so jumbo koi breeders and owners will invest much more time and resources into their koi collection than a domestic koi owner will.

Considering the fact that jumbo koi fish are bred from gigantic parents, even as juveniles, it is recommended that they be kept in appropriately sized outdoor ponds, fit for their adult size.

Conclusion

If you’re a first-time koi fish owner, the intricacies of getting them to grow faster might sound intimidating. In reality, they are one of the easiest to care for freshwater fish in their size category.

If you’re concerned with your koi’s growth rate, you now have a list of factors/tank conditions to optimize, and you’ll have an impressively-sized koi graciously swimming in your tank in no time.

Consistent water quality, a growth-focused diet, sufficient space, and low-stress levels will complement a good set of koi fish genes and make your pet koi dragon grow.

Recommended Reading:

• How Long Do Koi Fish Live? Lifespan, Care, And More
• How To Change An Aquarium Filter Without Losing Bacteria