Koi fish (Nishikigoi) are very attractive and are always kept for their colorful appearance. This species is a common ornamental pet fish among aquarium enthusiasts. Even though they originated in Japan, they are raised globally, So how fast do they grow?
On average, a healthy koi can attain a length of 0.5-0.7ft after 12 months. It can grow to 1.5ft or more after 3 years if well cared for. While they tend to have a life expectancy of 14 years, Japanese history proves that they can live up to 40 years.
Average koi growth rate
Koi fish grow at an average rate of 0.66mm per day. This translates to about one inch (2 cm) each month and 9 inches for the first year of their life. If a koi is less than 6 inches long after a year, its living conditions (diet + habitat) should be re-evaluated.
In contrast, your koi should be within 15-18 inches at the age of two years. Knowing the growth rate of your koi fish is important since it helps avoid overcrowding and helps determine the koi breeding, selling, and readings for competitions.
Below are some factors that will determine the average growth of a koi fish:
1. Age
Over 100 species of koi fish come in various sizes and colors. Nonetheless, we will focus on the average growth rate of these fish at different ages without concentrating on any species.
1-6 Weeks Koi
A baby koi grows at an average rate of 0.03 inches daily until 6 weeks old. Thus, your koi should be about 0.20 inches in its first week and 1.2 inches by the sixth week.
Typically, the baby koi appears brown during this age phase or slightly resembles the red or orange color identified in the adults. However, these colors won’t be as bright as they mature. They could also appear yellow, white, black, or gray.
2-6 Months Koi
At two months, your baby koi should be approximately 1.6 inches long. At six months, they should be as long as 5.1 inches. The koi coloring should continue to establish at this age. You might realize that the color will vary, deepen, and become more noticeable.
7-11 Months Koi
Past six months, the koi growth rate started to reduce to 0.2 inches daily until the second year. At the 7th month, your Koi should be about 5.8 inches long and continue growing constantly. The Koi’s color will vary, deepen, and grow brighter as they approach maturity.
1-Year Old Koi
Your Koi should be at least 9.2 inches in the first year of its life. However, it may be larger or smaller depending on the environment and its species. The fish will grow at a constant rate of 0.21 inches daily until it attains two years.
2-Year Old Koi
Your koi should be about 15.6 inches long at two years of age. At this age, their daily growth rate will continue to reduce as it ages. Typically, the koi’s color is in bloom by its second year.
3-Year Old Koi
For some species, koi will attain their adult size at this age. Your koi will be about 15-16 inches at this point, and they will continue to grow to 20 inches at three years old. Koi fish take about three years to become full-size adults.
A Summary Table Showing Koi Fish Size at Various Ages:
| Age | Size of Koi Fish (Inches) |
| 1 Week | 0.20 |
| 1 Month | 1.1 |
| 6 Weeks | 1.2 |
| 6 Months | 5.1 |
| 1 Year | 9.2 |
| 18 Months | 12.6 |
| 2 Years | 15.6 |
| 2.5 Years | 18 |
| 3 Years | 20 |
2. Pond size
Generally, adult koi require a minimum of 250 gallons volume for each fish. If you have a female able to spawn, you should look for 500 gallons for each female. However, a bigger pond will produce the best results.
You shouldn’t restrain a koi to a tank. Instead, provide them a home in a pond of at least 1,000 gallons with a depth of 3 feet.
3. Temperature
A high temperature leads to a fast metabolism, as the two are directly proportional. Thus, warmer conditions will facilitate the koi’s growth rate, while colder conditions lower its growth rate.
You may have witnessed your koi growing faster due to the warm condition in your pond. Nonetheless, scientific studies reveal that the ideal water temperature for koi’s high growth rate is about 75-80°F. Therefore, you can focus on maintaining this temperature to increase your koi’s growth rate.
4. Genetics
A discussion about koi’s growth rate would be comprehensive based on genetics. Like koi fish, some people in your environment are taller than others.
Thus, they possess different genetic compositions that affect their growth ability. Unfortunately, it can be challenging to identify the ideal variables that affect koi’s growth since various variables, such as genetics, are unignorable.
5. Water quality
The optimum pH level for koi’s growth is about 7.0 to 8.5. You should maintain zero ammonia levels since levels above 0.25 will risk your koi’s life. Also, you should keep a nitrate level within 20-60 ppm. Notably, you should maintain nitrites levels lower than 0.25 ppm, but the most optimum condition is zero nitrite level.
6. Nutritional blocks
Koi fish thrive well when provided with a suitable diet. You should always focus on ensuring proper storage of their food and disposing of any unsealed leftover food after six months, irrespective of its package date.
You should feed your koi fish with pellets containing high protein, minerals, and vitamins for the best outcome. Your fish will grow faster depending on the quantity and quality of the food you supply them. Like cichlids, koi fish also love strawberries.
7. Stress
You may have heard some little stress is good for you, but it is the opposite for koi fish. Stress, such as attack by parasites, poor water quality, bullying by other fish, extreme water temperatures, low amounts of oxygen, and insufficient food, can alter the growth rate of your koi fish.
Therefore, minimizing such stress on your koi would enhance their growth rate. At its worst, stress is one of the reasons fish may die.
Can I increase the koi growth rate?
You can increase the koi growth rate by:
- Providing enough space to prevent overcrowding.
- Similarly, you can offer them protein-rich foods and avoid feeding them with excess carbohydrates.
- You can also provide them with additional supplements, such as Quillaja, a growth stimulant.
Typically, you don’t need to feed your koi multiple times with less food. Always remember that these fish don’t have a stomach, so if they acquire a lot of food at once, they will release it partially digested.
Nonetheless, you can maintain a predictable and steady feeding schedule in your aquarium by using an automatic feeder. It minimizes the chances of guessing the amount to feed your Koi. Like Kuhli Loaches, koi also eat snails and many other foods.
- Not to forget, you can also increase the Koi’s growth rate by maintaining a temperature of 81°F. Scientific studies show temperatures between 75°F and 80°F are ideal for your Koi’s growth. However, a recent study has discovered that this fish will grow faster at 81°F.
Summary
Under ideal conditions, such as the right pond conditions and proper food, healthy koi fish will grow one inch per month during their first year. At three years of age, grown-in-captivity koi fish will grow up to 20 inches and more, depending on their genetic composition and species.
Nonetheless, you can increase the koi’s growth rate by providing ideal water conditions, temperature, and food requirements and keeping them in a large aquarium.
Hopefully, this post has answered your koi fish growth rate questions. Now, embrace your freshwater aquarium hobby!
References for this article include the Journal of Thermal Biology. Effects of temperature on the foraging and growth rate of juvenile common carp, Cyprinus carpio. ScienceDirect. Effects of ration size and temperature on the growth of juvenile common carp (Cyprinus carpio L.). Smithsonian’s Nationa Zoo & Conservation Biology Institute. Japanese Koi.
