It is nearly impossible to tell the sex of oranda goldfish until they are one year old and 6 to 7 inches in length.
The oranda goldfish is among the most popular aquarium fish in the world. Orandas are available in two color varieties — metallic and calico-based. The metallic oranda goldfish, as its name suggests, has a distinctive sheen and can be purchased in many different colors. The calico-based variety have a bluish-tone and are covered in spots. Sex can be determined by comparing the size and features of the the fish in your tank. If you do not have multiple orandas, the sex can only be determined by examining the fish closely when the water is warm enough to encourage breeding.
Compare the sizes of the oranda goldfish in your tank. Males are almost always smaller than females. Size comparison alone is not foolproof, but it does give an indication if you are trying to determine a fish's sex outside of the breeding season. If you do not have multiple fish in your tank you will need to use other methods to determine sex.
Examine the shape of the vent area, which is the opening under the tail fin. The vent of female oranda goldfish is usually rounder and more pronounced than that of the male.
Compare the pectoral fins, which are the small fins located just behind the fish's head, as they are almost always noticeably thicker in males than females.
Look closely at the head and gill covers for small white bumps, known as breeding tubercles, that develop on the male oranda goldfish during the breeding season. Breeding is triggered when the water temperature warms to between 50 and 78 degrees Fahrenheit.
Examine the fish from above the water. During the breeding season the female's body is significantly wider than the male's, due to the eggs that she is carrying. If you do not have multiple fish in your tank, examine the fish from above regularly before breeding season begins. Once the water has warmed to the necessary temperature you should be able to see a difference in the size of the female's abdomen as she prepares to spawn.
Watch for unusual behavior. When the female is preparing to spawn, the male oranda goldfish will begin to nudge the vent of the female and chase her around the tank, bumping her repeatedly.