Instructions
Note your goldfish's overall build. As a general rule, a male goldfish will be thinner and longer than a female of the same breed and variety. Females tend to be plumper and more compact. However, a fish's build alone can not reliably indicate its sex.
Look at the goldfish's gill area. The gill covering is most likely the same color as the scales on the surrounding parts of the fish's body. When ready to spawn, a male goldfish may have white spots peppering a portion of the gill cover. These white freckles, called tubercles, appear on a male's gill covers only when he is ready to spawn. Although visible tubercles positively identify a goldfish as male, the absence of tubercles does not automatically identify the fish as female.
Observe the fish's abdominal area for signs of swelling. A mature female goldfish will have a noticeable plump belly that swells periodically when she is ready to lay eggs. This does not mean that she will necessarily lay fertile eggs, only that she is capable of producing eggs. Males may also develop abdominal swelling due to overfeeding or disease.
Monitor your goldfishes' behavior for several days, noting the individual behavioral patterns of each fish. In general, a goldfish that chases others is male, while those he chases are most likely female. Although occasional territorial disputes do occur between members of the same sex, repeated chasing is often a sign of courtship.
Look at your goldfish's anal opening, or vent. This is the area from which a mature fish will expel sperm, eggs and feces. If the opening is visibly convex, extending outward, your fish is most likely a female. Mature males tend to have concave or flat vent areas.
Examine the anal fin of your goldfish, which is the fin on the underside of the fish directly in front of its vent area. The first ray of the fin -- a spinelike structure on the fin's underside -- is usually thicker in males and thinner in females. In mature males that are ready to spawn, the anal fin may also display tubercles similar to those seen on their gill plates.
How to Tell the Difference Between a Male Goldfish & a Female
Some species of ornamental fish, such as Siamese fighting fish and livebearers, are extremely easy to sex. However, most members of the minnow family, including the common goldfish, display only subtle exterior differences between sexes. Several minor differences, however, can help you to identify the sex of a goldfish in your care.