Goldfish eggs hatch within 48 to 72 hours, and the fish may live as long as 15 to 25 years. Within a week, the young start to resemble goldfish more closely, although they don't develop mature goldfish coloration until they are approximately a year old. Goldfish only reach sexual maturity if provided with the right water and nutrition; breeding is triggered by a sudden change in temperature.
Because goldfish are cool-water fish and can grow extremely large, they require more oxygen in the water than tropical fish. Rate My Fishtank recommends purchasing a tank longer than it is wide, keeping in mind how many goldfish may eventually live there. The larger tank surface area also allows for a better exchange of gases between the air and water. If a goldfish lays eggs, remove them to prevent the parents from eating them. If raising young goldfish fry, don't be surprised if they grow and change rapidly during the first weeks of development. This is an adaptation that helps them survive the risk of being eaten by adult goldfish, insects and other fish. Some goldfish can't breed on their own and must be bred through a risky method called hand stripping. If done incorrectly, this method can result in injury to the goldfish.