In an ideal environment, an adult koi can reach a whopping 3 feet long.

Under most conditions your finned koi will not grow to that immense length, but there are ways to reach its full potential. Koi will keep growing throughout their lives. 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.

• Water temperature. A steady 75° F will keep koi in a more active growth state where they’re building muscle and body mass.

• 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.

• Genes play a huge role in how large a koi could grow in the right conditions.

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.