Fish add movement and character to any pond, and they are an important component in a healthy aquatic ecosystem. They also eliminate mosquitoes and other insects that breed in water. Many fish species also eat aquatic plants, however, so it is important to keep their numbers at a reasonable level. Many aquatic gardeners keep ornamental carp known as koi in their ponds. These fish are available in a myriad of colors and patterns and can live for a very long time and grow quite large. Goldfish are very adaptable and can survive the winter in your pond if it does not freeze to the bottom and a portion of the water surface is kept free of ice to allow for gas exchange.
If you choose to keep fish in your pond, make sure they don't have the opportunity to escape to natural waterways in your neighborhood where they can compete with and displace native fish species.