Instructions
Behavioral Signs
Observe several goldfish in an aquarium together. Make sure that there are at least a dozen to virtually assure that there will be a combination of males and females.
Watch for fish chasing others. When females are ready to breed, they release a chemical that attracts males, causing them to give chase. Territorial fish can also act aggressively, so this is no guarantee of the fish's sex.
Watch the chasing fish to see if any are nudging the anal vent of the fish they are pursuing, which is located at the base of the tail. If so, the pursuer is a male.
Physical Signs
Look for breeding stars, or breeding tubercles, on the gill covers of the fish. The gill covers are the large, circular scales overlapping the gills. Males will develop these tubercles, which look like shiny, white spots, when they are ready to breed.
Males tend to have more slender bodies than the females, which can be identified, particularly during breeding season, by bulges in their stomachs.
Examine the fish's anal vent. In males, the vent will be slightly longer and curve inward, whereas in females, it will be smaller and protrude outward.
How to Tell if a Goldfish is Male or Female
Sexing fish, or identifying the gender, is a task that varies in difficulty depending on the type of fish. Some, like bettas and swordtail fish, are easy to distinguish by the lengths of their tails. Others, like goldish, require a great deal of observational skill, patience and a dash of expertise.