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?