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I started off with 4-inch koi and now they’re 8 inches. How big will they get before they stop growing? | Decorative Ponds & Water Gardens Q & A


Q: I started off with 4-inch koi and now they’re 8 inches. How big will they get before they stop growing?

Q: I started off with 4-inch koi and now they’re 8 inches. How big will they get before they stop growing?

Lauren – Lincoln, NE

A:  In an ideal environment—a supersize space with pristine water conditions at a stable 75 degrees Fahrenheit—an adult koi can reach a whopping 3 feet long. A yard! Nearly a full meter! Yep, that’s a really big fish.

Don’t worry: Chances are slim that your finned pal will grow to that immense length, but you can help it reach its full potential. Koi will keep growing and growing throughout their lives, sometimes faster than others. How fast and large they grow depend on several factors, including:

  • Water quality. Clean water pumped full of oxygen will promote a fish’s health and growth, while poor water quality can stunt its growth. Koi will tolerate a dirty, cloudy environment, but their development and vitality will suffer. Make sure you have an aeration kit in place and are using beneficial bacteria from the DefensePAC® to keep the water crystal clear.
  • Water temperature. A steady 75° F will keep koi in a more active growth state where they’re building muscle and body mass like crazy. Cooler or fluctuating water temperatures trigger a slower growth rate as they slow down their metabolism and activity level. If you live anywhere other than in the tropics, expect to see slower growth in the winter months.
  • Nutrition. Food—and how much of it they eat—matters. Fish food that’s packed with protein and vitamins is formulated to help koi grow and develop. Some also contain ingredients that boost your fish’s vibrant colors.
  • Genetics. Genes play a huge role in how large a koi could grow in the right conditions. Colossal parents often produce colossal offspring, and if those fry live in a spacious pond with clean, aerated water and good food, who knows how big they’ll get!
  • Age. Like most living things, koi develop faster when they’re young and slower as they age. Your 4-inch koi quickly doubled in size because they’re still adolescents; as they get older, their growth rate will slow down.

If you have big plans for your koi, give them an ideal ecosystem and good grub. Your colorful friends may not reach that 3- or 4-foot mark—but you never know!

Pond Talk: What’s the largest koi you’ve ever had—or seen in person?

Energy Efficient Design For 24-7 Use - The Pond Guy® Water Garden Aeration Kit

4 Responses

  1. It is true your koi can get 3 feet and bigger depending on the kind of koi you have. My koi are 20 years old and are a little larger then 3 feet. When they came from Japan they were maybe 6-8 inches. Water quality and food has everthing to do with itm these guys have lived 20 years in a crystal clear pond they have only eaten Hakkari food and fresh fruit in the summer. They have never been treated with chemicals everything has been done naturally. My koi are so time spent that they come to they edge of the pond when they hear your voice and they eat straight out of your hands. They love for you to pet them that they will roll so you pet them all over they love it! You almost forget their koi.

    • That’s wonderful Stacey, thank you for sharing your experience. Does your pond have a lot of plants to help keep the water clear or is it in a shaded area?

  2. Well if you have you Koi in a deep water pond that they can dive deep down and swim in they love that. Mine are just over the size of a yard stick. Mine came from Japan. The man that owns the Koi shop where mine came from they have one there that it takes 2 men to transport it. One man can do it but hes just so big and their afraid of dropping him. Ive flown my Koi from Calif to Florida. But they went from a 2,500 to a 38,000. So they moved into a palace they though lol.
    Kois love perfectly clear water. They love fruit. It great for their color. If you want them to grow give them room to swim. Feed them remember one thing about Koi they will eat and eat and eat they dont get that they are full. So if you want to fatten up your Koi and make them grow feed them extra. Keep the water clear!!

  3. Most of our koi are 10 yrs. old and are in the 2 ft. range. We have cold winters and hot summers and they seem to be happy no matter what the conditions. We even moved them when we built a new house. That was an interesting project!

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