The common aquarium goldfish, Carassius auratus, can live thirty years or more under ideal conditions.
An aquarium limits their size, but a pond lets them achieve their maximum growth. Some goldfish can reach up to two feet in length.
Goldfish often change color as they age and develop their full color patterns.
In fact, one of the skills of goldfish breeders is to predict which fish are going to develop the best colors.
Goldfish are closely related to the carp family and are quite hardy. As long as the pond does not freeze over completely they can withstand very cold temperatures.
Another pond fish, the koi, is even larger and more elaborate.
It is very closely related to the wild carp, Cyprinus carpio, but has long been bred to achieve fantastic colors and shapes by fanciers in Japan, where prize fish are auctioned for large amounts of money.
The koi is long-lived but not as hardy as the common goldfish.