If you or your child is eager to name the new family fish, it will be helpful to know its sex. Knowing the sex of your goldfish can also help you if attempting to breed them. If you want to buy mature goldfish that are ready to breed, it will be easy to select a male and female. Before goldfish mature, however, it is much more difficult to tell which sex they belong to.
When both goldfish are swimming through the tank or bowl, take a bird's eye view of them. Mature goldfish will have a distinct body shape at maturity because the female needs to be wider and softer to carry eggs. Male goldfish at maturity will retain a thinner, torpedo shape that is characteristic of both sexes when young. Female goldfish may also appear larger on one side of the body rather than plumper overall.
The fins, gills and vents are the only truly defining characteristics on any goldfish. Under and behind the gills of the goldfish, you will find its fin. On a male goldfish, the fin will be thicker, pointier and more rigid than the female's. In comparison, the female's will be shorter and rounder. The vent can be found at the back of the fish and is the anal opening. When a female fish is ready to breed, her vent will become larger and rounder, from concave to convex. The male's will remain small and concave.
Only the male will develop new growths when ready to spawn. This growths are white bumps called tubercles. You can find tubercles on the male's head, gill covers and pectoral fins. The fins develop tubercles only when the fish have begun to breed, however.
Male fish are aggressive when it comes time to breed. A male fish will chase the female around the tank or bowl to initiate breeding. The male may go as far as knocking against the female to force release of the eggs. You will find that this chasing is relentless, making it an easy behaviour to catch and observe.