How to Tell a Goldfish's Age

Although most species of aquarium fish can be aged based on how long they are, this doesn't always hold true with goldfish. That's because a goldfish will stop her growth based on the size of her environment. She's unlikely to reach her 12-inch maximum size if you keep her in a 20-gallon tank, especially if she shares the space with goldfish tank mates. That's because growth-inhibiting hormone builds up in the environment, limiting her size.

However, size isn't important to goldfish longevity. All of the world's oldest fish were kept in small environments that stunted their growth, linking small size with longer life. Conversely, fish who live in higher water temperatures, eat ample food, and swim in lots of space tend to live 10 years or less of a goldfish's potential life span of 20 to 30 years.

If your fish does have ample space to grow, you can expect she'll do most of her growth within the first year of life. How big she gets also depends on how she was treated early in life. Goldfish breeders and pet shops sometimes keep fish in overcrowded tanks, stunting their growth from the start. If you're raising fry yourself, you can influence maximum growth by providing quality nutrition without overfeeding, giving her enough space, and performing frequent water changes.

It's easy to judge the age of juvenile goldfish by physical development. During their first week of life after hatching, the fish grow from about 4.5 mm (3/16 inch) to 5 mm. They average increasing their length by 27 percent per week during the first eight weeks of life, nearly doubling their weight weekly. Look for the largest growth spurts during week two and three, when fish increase in size by 40 percent and 57 percent, respectively.

At less than 8 weeks old, you can easily determine goldfish age since they don't have fully developed fins. At 1 week of age, the pectoral fins are most obvious, but they also have a small medial fin running from where their dorsal fin would start around the end of their tail to the vent area. The tail fin develops in the second week, and dorsal fins during week three. By 1 month old, their dorsal fin is well developed, and the anal and ventral fins appear. All fin development is complete by 5 weeks old, when the fish is about 11 mm (5/8 inch) long.

By the fifth week, features such as protruding eyes will start becoming obvious. The goldfish will lengthen to 1.5 to 2 inches by 6 months old and become an adult goldfish at the age of 1 year at approximately 3 inches long. Given optimal water parameters and nutrition, a goldfish will continue growing for the first seven years of life until he reaches about 12 inches long.

When your fish is an adult goldfish in her prime, the only way to tell her age is through her scales. Tiny rings develop on the scales for each year of life, somewhat like a tree ring. The process involves removing 10 scales from your goldfish's body and examining them under a microscope. Because they can be tiny on small fish and rings are present primarily in fish who endure temperature extremes outside, you should consult your veterinarian before considering this method.

Goldfish color can also be a clue to age. Young fish are more of a bronze color. Goldfish in the prime of life are orange (unless they're a black moor, marble, or other color variation). Look for signs of your goldfish entering old age, such as fading color. Some goldfish even become silver or white in their later years, much like humans getting white or silver hair.