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Home > Fish > How Fast Do Koi Fish Grow? Vet-Reviewed Growth Rate & Care Info!

How Fast Do Koi Fish Grow? Vet-Reviewed Growth Rate & Care Info!

koi fish in pond

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Koi are a popular freshwater pond fish that can grow very fast throughout their development. Koi are commonly kept in freshwater ponds and have an average lifespan of 25 to 30 years depending on their living conditions. Koi fish come in a variety of different colors and shapes, but they all have one thing in common—they are exceptionally fast growers. The average growth rate of 0.8 inches (2 cm) per month up to 36 inches (91 cm), their growth depends on their diet and living conditions.

Many aquarium keepers are surprised at the fast rate their koi fish grows, which creates a lot of confusion over the correct size pond or tank these large fish need. If you are looking to purchase koi and wondering how big these fish can get, then this article will give you all the information you need.


Do Koi Fish Grow Quickly?

Yes, koi fish are fast growers, with an average growth rate of 0.8 inches (2 cm) per month. The growth rate of your koi fish will depend on the variety, diet, and living conditions of the fish. Koi grow the most within the first 2 years of their life and their growth rate will slow down to 0.4 inches (1 cm) per month until they reach their full adult size.

The growth does not only affect the koi fish’s length, but also the width of their body. Long-finned koi varieties may appear larger because of the unusually long fins that span out from their body. Not all koi will grow at the same rate, as some koi will grow faster than others depending on their genetics and living conditions in comparison to other koi.

Young koi grow the fastest when their conditions are ideal, and their growth will start to slow down after several years. However, like all fish, they never truly stop growing.

How Big Do Koi Fish Grow?

A fully matured koi fish can reach up to 36 inches (91 cm) in length and weigh up to 20 pounds. However, not all koi fish will reach such a large size as the average koi only reaches between 20 to 24 inches (50 – 60 cm) and weigh between 9 to 12 pounds. The adult size of your koi will depend on the variety, as some koi varieties can grow much larger than others due to genetics.

Koi fish
Image Credit: Piqsels
Let’s look at the average size of different koi:
  • Domestic koi: 12 – 17 inches (30 – 43 cm)
  • Japanese koi: 22 – 26 inches (56 – 66 cm)
  • Butterfly koi: 24 – 30 inches (61 – 76 cm)
  • Jumbo koi: 32 – 36 inches (82 – 91 cm)

Koi fish that grow genetically larger than other varieties may have a faster growth rate than other koi, however, the growth will slow down as they begin to mature.

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What Influences a Koi Fishes Growth?

Certain factors can influence the growth rate of koi fish, and some koi varieties that can grow large may fail to reach their average adult size if these factors influence their growth.

Pond Size

Koi fish cannot grow large if they are kept in a small pond or aquarium. The size of the pond will also play a role in the water quality because small bodies of water that are overcrowded with koi fish will become dirty quickly which can affect the health and longevity of the fish. Since koi are large fish that grow quickly, it is important to raise them in a large pond with filtration and aeration to keep them healthy.

Koi Fish
Image Credit: CC0 Public Domain, Pxhere


If a koi fish does not have the right genetics to grow large, then even the biggest pond, healthiest diet, and clean water will not change their maximum adult size. Certain varieties of koi have the genetics to grow large, such as the butterfly and jumbo koi which can grow to an average size of 30 inches (76 cm).

Stocking Rate

Overcrowding your koi in a pond or aquarium means that there is less space for the fish to swim around in. The bioload in the pond will also increase which will negatively impact your koi and may cause unnecessary stress that can make your koi prone to illnesses. Stocking your koi fish’s large pond correctly is just as important as making sure the pond is large enough so that each koi has enough space to grow and move freely.

Tancho koi
Image Credit: ClickMood, Shutterstock


Koi will grow the most within the first 2 years of their life, and it will steadily decline as they mature. If a koi fish was not growing well during its prime growing years, it can be difficult for the fish to grow any larger especially once they are more than 10 years old.


Koi fish need a healthy and balanced diet to provide them with the proper nutrients to grow properly. Young and growing koi fish will consume more food because they need extra food for development. Proper nutrition is essential throughout the koi fish’s life, but extra vitamins and minerals can benefit koi during their development stages.

Water Quality

Water quality is another important aspect to consider if you want to accelerate your koi fish’s growth. The water parameters (ammonia, nitrite, and nitrate) play a role in your koi fish’s health and growth, however other water quality aspects such as the pH, alkalinity, temperature, and dissolved oxygen in the water affect your koi fish’s growth too.


How Long Does It Take For Koi To Grow?

The average koi fish can live between 20-35 years of age and their growth will usually slow down after 3 years. Koi fish may continue to grow slowly after this stage and grow between 4 to 6 inches more each year until they reach their final growth stage at 10 years of age, however, koi never truly stop growing and will slowly keep growing until they die of old age.

Kohaku koi fish
Image Credit: Gatot Adri, Shutterstock

Why Do Some Koi Fish Grow Slowly?

Some koi fish will grow slower than others, as genetics play a major role in your koi fish’s growth aside from diet and pond size. Stress or sickness may be causing your koi to develop slowly, and this can be brought on by poor housing and water conditions.

Koi require a large pond, the right diet, and appropriate stocking numbers to prevent their growth from being stunted. A stunted koi fish will fail to grow to their appropriate size due to genetic aberration, but malnutrition and poor water quality in a small environment may cause your koi’s growth to become stunted.

How To Make Your Koi Fish Grow Properly

If you want to raise healthy young koi so that they can grow quickly without hindering their growth, then these tips can be helpful:

  • Keep your koi in a large pond that is around 3 to 5 feet deep.
  • Ensure the pond has a good filtration and aeration system to improve your koi fish’s water quality.
  • Avoid overstocking the pond with too many koi fish so each fish has enough space to move and grow in.
  • Feed your koi a balanced diet specifically formulated for koi fish with vitamins and minerals to help with their development.
  • Feed young koi appropriately sized portions of food 2 to 4 times a day.
  • Maintain the ideal water chemistry for your koi fish.
koi fish swimming
Image Credit: Pexels, Pixabay

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Final Thoughts

The fast growth rate of koi fish may intimidate many new fish keepers, but once you set up the correctly sized pond with the right water conditions and diet, you will see that your koi fish’s growth will remain steady throughout the first years of their life, and slow down once they reach 10 years of age. Koi fish will grow at different rates, but most of their impressive growth will be down within the first 3 years of their life.

See also:

Featured Image Credit: Vital Safo, Shutterstock

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