How Fast do Koi Fish Grow?

Koi is a variety of carp that can be found in ponds and aquariums all over the world. They are often sought after for decorative purposes due to their colorful appearance.

Koi are domesticated too and are considered to make excellent pets. When young they can be kept in a spacious indoor aquarium. As they begin to grow they may require more space in the form of an outdoor pond. 

If you are looking to keep koi, you are likely wondering how fast these fish grow. Below, we have identified the size that your koi fish are likely to grow too, and various factors that can affect their growth. 

There are around 100 different types of koi fish that can be identified by their colors and body pattern. The growth can differ depending on the type of koi.

Koi fish are generally expected to grow around 4 to 6 inches in length every year which is equivalent to around 2cm a month. Although the growth time can vary, it usually takes around 3 years for a koi fish to reach its full size. Some can grow to as much as 9” in their first year of life. 

Some types of koi can grow to reach around 2 feet in length. The majority of domesticated koi tend to reach around 12-15” in length.

As the size can differ between the types of koi fish, it is important to ensure that the aquarium where you intend to keep them is spacious enough for their size. A young koi fish isn’t going to be the same length as a fully mature koi fish so it is necessary to bear this in mind. 

What Factors Affect Koi Growth?

Several factors can influence koi growth from the climate, the conditions inside of the tank, and feeding. We have identified some of these factors below. 

The Temperature

A temperature of around 73 to 75 degrees Fahrenheit is considered to be the best for koi growth. They tend to thrive in this temperature of water as it can help to maintain healthy metabolism levels.

The temperature of a pond in the summer tends to remain within this temperature range which is ideal. If you intend to keep your koi indoors, it is important to ensure that their aquarium features a heater as this can help to maintain this temperature.


The diet of your koi fish can be highly influential upon their growth potential. Your koi fish should be fed a high-quality diet that consists of flake or pellet food. However, to add variety to their diet their standard diet can also be supplemented by vegetables, fruit, and shrimp. 

The Age

When considering growth it is important to account for the age of the koi. Koi tend to grow the most during the early stages of their life and their growth tends to slow down as the fish get older.

If you have a young koi you are going to witness it grow as it ages. However, if you get an older koi, it is going to have less growth to do. 

The Size of the Pond or Aquarium

If your koi fish are kept in a pond or aquarium that is too small, of course, this is going to hinder their growth rate. While it won't necessarily stop them from growing, it will delay the process as they aren’t going to have the ability to move around the pond or aquarium freely.

The Number of Fish in the Pond or Aquarium

This is closely linked to the previous factor. If you have too many fish inside the pond or aquarium then this is going to impact their growth. Your koi fish are going to be restricted on the space that is available and as a result, they aren’t going to be able to grow to their full potential.

If you are unsure as to how many koi should be kept inside a pond or aquarium, it is recommended to ensure that you don’t have any more than 1 inch of koi per gallon of water. Having more than this may see you with an overcrowded koi pond. 

The Quality of the Water

The environment that your fish are kept in is also going to affect their growth. Your koi will grow to its greatest potential when kept in a healthy pond or aquarium.

The pond needs to be maintained and cleaned regularly as this will help to remove the dirt. The water needs to be aerated with dissolved oxygen and a 7.5 pH rating.

Keeping your koi in an unhealthy environment can have negative impacts on their growth. If they are swimming in a dirty pond or aquarium their health is likely to deteriorate and so they are going to struggle to reach their full size.

You may choose to partly clean the water weekly as regular whole water changes can cause your koi to become stressed. 

Koi Genetics

It is important to consider the genetics of koi fish, while they tend to be very similar in color, pattern, and size, this isn’t to say that every koi fish is going to be the same. Some koi fish may be smaller than others when they have reached their full growth potential.

Final Thoughts

Koi fish grows the most during the earlier years of their life, with the growth rate slowing down as they get older. While you should expect to see your koi grow to around 12 to 15”, it is important to remember that not all koi fish are the same.

Some may grow bigger than this while others may be a little smaller. The size can also differ slightly between the various types of koi. There are many steps that you can take to help your koi grow while benefiting their health too.

A healthy koi that is kept in a spacious pond or aquarium with little overcrowding, good water quality and a high-quality diet is likely to grow more than those who are restricted in space and kept in dirty water with unhealthy diets.