It's difficult to tell the sex of a goldfish, because the fish do not have visible external sex organs. Instead, you must read more subtle cues from fishes' behavior and appearance to ascertain whether they are more likely male or female.
Age and Breeding Season
It's almost impossible to tell the sex of a goldfish before the fish is of breeding age. Usually, goldfish reach breeding age when they are about 12 months old. The easiest time to tell the gender of an individual fish is during the breeding season, also known as "spawning season."
Breeding Season Characteristics
During the breeding season, male goldfish have breeding tubercles on the covers of their gills, and along the edge of their pectoral fins. Male breeding tubercles look similar to white pimples. A female goldfish in breeding season will have a noticeably deeper body shape than a male fish, as her body is filling with roe or eggs.
There are several subtle physical differences between male and female goldfish. Female goldfish tend to be more brightly colored than males. Male goldfish have a pronounced ridge that runs through the rear of the pelvic fins and down to the opening of the fish's vent. Females have a smaller version of this ridge, or may have no ridge at all.The abdomen of a male goldfish usually is noticeably firmer than that of a female goldfish. If you are able to get a look at the vents of the fishes, those fish with a convex, round vent most likely are female. Male goldfish have a relatively concave, slit-shaped vent. Finally, the pectoral fins usually are more pointed in shape on a male goldfish than a female.
You may be able to guess the gender of a goldfish by observing its behavior. In general, female goldfish tend to be more active than their male counterparts. During breeding season, male goldfish tend to be active and persistent in chasing and nudging female fish. Some breeders believe that you can tell the sex of goldfish by releasing a fish known to be female into the tank. The male goldfish will chase the female, while other female fish will ignore her. However, male goldfish often chase one another, so you must be completely certain the test fish is female before using this method.
Jae Allen has been a writer since 1999, with articles published in "The Hub," "Innocent Words" and "Rhythm." She has worked as a medical writer, paralegal, veterinary assistant, stage manager, session musician, ghostwriter and university professor. Allen specializes in travel, health/fitness, animals and other topics.