How Long Do Koi Carp Take to Grow & How Big Can They Get?

Andrew HodgesKoi Carp FactsLeave a Comment

Hungry Koi how fast and big can they grow

After keeping Koi Carp for over twenty years I have seen massive differences in fish size and growth rates, but I have some neat tips on getting the best growth rate from your Koi.

Koi can grow at immense rates in the correct environment. In Japanese mud ponds during the warmer months, for instance, they can grow over 30cm in one season. In the UK however, cooler water temperature reduces this dramatically to perhaps a maximum of only half that growth, ie 15cm or less.

So how do we optimise the growing environment and get the best out of our baby koi carp? In this post, I will explain how the professionals do it and how we can take advantage of this knowledge.

How do breeders produce huge and beautiful Koi Carp

During the warmer months, Japanese Koi breeders select very high-quality Tategoi (one-year-old Koi Carp) from their stock, place into mud ponds, so that they can maximise their growth.

The uniquely fascinating qualities of the mud ponds produce extremely large, beautiful and much sought after Koi Carp.

The koi spend around eight months in these mud ponds after which the breeders drain the basins and harvest the fish. Harvest season is during October and November each year.

Why grow fish in a mud pond?

Mud ponds are lined with mineral rich, natural clay and filled with millions of gallons of pristine local water sourced from the surrounding mountains. During the growing season, water is being continuously drained out of the ponds and replenished with fresh mountain water.

Fish produce Pheromones naturally. The Koi have a natural resistance to growth where pheromone levels are high. In a sense, the fish know innately not to overstock or outgrow their environment. One of the reasons for changing the water in the pond is to dilute or remove the pheromones encouraging growth.

There are no human-made filters in the mud ponds such as you would find in a hobbyists pond, so the water changes also remove pollutants, such as ammonia, nitrate etc.

What is the perfect water temperature for growing Koi Carp?

During the growing season in Japan, water temperatures can reach 84 degrees Fahrenheit. However, 73 degrees Fahrenheit is considered optimum for growth. Food is in abundance, both naturally occurring within the ponds and also provided regularly by the breeders in pellet form.

Mt. Fuji in autumn

In the UK many fish dealers note that heating their growing on ponds to 73 degrees Fahrenheit does indeed seem to provide the optimum temperature for maximum growth. Any higher and the fish become less interested in food and are not exercising as freely.

What can I do to grow big Koi Carp?

If you are looking to grow your koi carp to their maximum potential, emulating the impeccable standards of the Japanese mud pond would serve you well.

My top 9 tips for growing huge Koi Carp:


1 Select great starter fish

When selecting Koi from the dealer to grow on, look for Koi with large heads, broad bodies and a muscular tail.

Nature has designed these fish to be the optimum eating machines. They will be the first to the food with big mouths allowing them to eat plenty of food and therefore grow as quickly as is possible.

2 Install an oversized filter

Make sure you have an oversized pond filter, one much bigger than the size your dealer told you would need to cope with your size pond.

To grow, we are going to feed a lot of food, with resulting waste products.

To maintain optimum water quality waste must be removed efficiently . The variety of different filters are discussed in this post here.

3 Heat the water

If possible maintain constant water temperature during your growing season, 73 degrees Fahrenheit is ideal.

4 Maintain constant water changes

Maintain a constant, slow water change, a dripping hose into your waterfall or filter. A water change at a rate of five to ten per cent of the total volume per day is ideal.

Do not undertake massive, one-off water changes, as this will stress the fish. Ensure that water used is either filtered or treated with appropriate water conditioner.

Vector illustration Japanese carps. Traditional Japanese symbol carps koi. Advertisements, signs, stickers, web banners, signage. Isolated on a white background.

5 Use clay supplement

Add a clay supplement to the water regularly – tap water is devoid of many minerals. Bear in mind that water changes will be removing this from the water too.

6 Provide the fish with some exercise

Use water pumps to provide a strong current so that fish have to swim to stay in one place. This way they will be healthier and maintain excellent body shape.

7 Introduce plenty of oxygen

Provide ample oxygen with air stones, waterfalls or fountains, ensuring perfect health. This is especially important at warmer water levels.

8 Rest the fish in the winter

Allow a rest period each year where fish are not fed or encouraged to grow. The naturally colder months of December and January usually surface. This rest period allows the Koi’s colours and skin to develop.

9 Feed, Feed, Feed

Most importantly feed appropriately sized, high protein growth food in small regular intervals. I use an automatic feeder for this and would recommend this to you. We aim to feed 3% of body weight per fish per day divided into five meals every day.

Important to note that as the fish grow so does the 3% 🙂

So what can I expect in terms of koi growth per year?

During year one you could expect massive growth, the first year is when most Koi Carp do their growing, as much as 20cm to 25cm could be possible.

In years two to four, you could expect 15cm further growth. From five years and onwards things generally slow down, with perhaps 2 to 3cm increase a year until the maximum size reached.

Genetics come in to play, as, in any family, there are differences in size and height. Koi Carp are no different; not every Koi has the potential to grow into a Jumbo Koi.

Largest koi carp on record

In the 1980s in Lake Biwa in Japan, a Koi Carp of almost 2 metres was caught and netted. The intention was to display this specimen in an aquarium in Kyushu. Unfortunately, the shocked fish died in the aquarium shortly after capture.

A British Koi dealer took delivery of a 1.2-metre fish in 2007 that tipped the scales at a staggering 41 kilos. This colossal fish named The Big Girl was more than three times the size of any other carp in the UK at that time.

The owner, Geoff Lawton, bought the her from a specialist Japanese breeder.

What’s unique about the mud in Japanese growing ponds?

Each year Japanese breeders replenish the clay lining of their mud ponds. Montmorillonite is the preferred type of clay.

Calcium-laden clay rich in minerals aids enzyme production and is heavy in nutrients together with the micro-organisms which thrive on them.

icroorganisms are part of the Koi Carp’s diet and one of the reasons for improved growth, health and colour gained in the mud ponds.

Clay is believed to possess natural detoxifying qualities. These regulate water quality naturally through a process known as flocculation, the negatively charged ions in the clay attach to the positively charged elements like ammonia and other dissolved organic compound.

I discuss this further in my post on Anoxic Filtration method, which you can add to your koi pond.

What is the oldest Koi Carp on record?

The oldest known Koi Carp was a female Koi named Hanako (Hah-Nah-Koh) or Flower Maid. The average life span of Koi is approximately fifty, with occasional specimens living to 100 years of age.

beautiful Japanese pond

Hanako, however, lived to the ripe old age of 226 years! She was born in 1751 and died in 1977 weighing 7.5 kg and having an overall length of 70cm.

Using a microscope, Professor Masayoshi Hiro counted the number of rings on Hanako’s scales to determine her age with relative accuracy.

She spent her life in a lake at the base of Mount Ontake, also referred to as Mount Kiso Ontake, the 14th highest mountain in Japan with five other Koi.

The five other koi also lived past one hundred years of age. Knobody knows the exact secret to the longevity of these fish; however, the crystal clear, fresh mountain water, mud pond, genetics, constant care all probably played their part.

So there you have it, my take on getting small koi carp to reach their full potential and become fantastic, healthy adult koi. Thank you for reading.

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