Thoroughly research your goldfish breed to accurately determine its age.
Goldfish are common aquarium fish available in a wide variety of colors and striking body patterns. These beautiful fish are resilient and can survive in water temperatures ranging from 40 to 90 degrees F. With proper care, some breeds of goldfish can survive for up to 30 years of age. If you plan to breed goldfish, you must determine their age to ensure that you are breeding adult fish and not juveniles. Most breeds of goldfish demonstrate several physical changes which can help you determine whether they are immature or adult fish.
Observe the color of your goldfish for signs of maturity. Most breeds of juvenile goldfish are generally brown, and change to a golden color when they become adults. The color change can occur at one year of age or when the goldfish grow to around 5 inches in length.
Determine the size of your goldfish. Mature common goldfish and comet goldfish can reach a length of around 5 inches within two years, while fancy goldfish attain a maximum length of 5 inches within 30 months. Though the maturity age varies for different goldfish types, they achieve an adult size between 10 and 36 months of age.
Look at the tail fin or caudal fin of your goldfish. The tail fins in young goldfish are round, while those in adult goldfish are sharp and forked. Additionally, the tail fins grow in length as the goldfish matures in age.