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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?

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6 Responses

  1. i have had several Koi reach almost 30 inches, and I have found that they got their best color when the water was not perfectly clear.

    • Hi James – Thanks for sharing. Do you have any thoughts on why the color is better with less clear water?

  2. Choosing Koi, truly is a commitment. When we started our pond, I chose ‘goldfish’ varieties that would not grow longer than 8″ (primarily because I knew we would have to house them indoors at least the first winter). This is their 3rd summer in the pond; after surviving their first winter. We began with 8 fish about 1 – 1.5″ in length; 5 (all fantails) survived the first summer and are still growing strong; they are about 5″ long.but, unlike Koi, I can’t train them to come to food, they flee to the bottom(about 5″ deep) when someone approaches the pond, They were “friendly” and playful, greeting me when I came home (likely just looking for food!) indoors, when I came home from work but lost that immediately outdoors. I may just enlarge the pond and add Koi, when I am ready to make a 20+ year commitment because I love the “personality”!

    • Hi Catherine – We are so glad you are having success and enjoying your fish! Don’t give up on the goldfish, they may not be as personable as koi but I’m sure they still appreciate the wonderful home you’ve created for them.

  3. How long can a Koi live?

    • Hi Ron – There are several factors that will determine how long a koi can live but in the right conditions it wouldn’t be uncommon for them to last 20-30 years.

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