Instructions
Observe how the goldfish behave around one another. If you are trying to determine gender in the pet store before purchasing a goldfish, you can observe which fish is chasing which. Male goldfish have a tendency to chase the females and nudge at their rear.
Look at the shape of the goldfish. A male will usually be longer and thinner whereas a female has a thicker appearance.
Check for a midline ridge on the goldfish's underside. Males have a midline ridge on their underside beginning from the back of their pelvic fins and ending at their vent. Females either have a smaller ridge or no ridge.
Study your goldfish's vent. A females anal vent will protrude slightly and her anal fin is thicker.
Wait until your goldfish are ready to start spawning. While this may be inconvenient for naming purposes, the differences between male and female goldfish are more obvious when they are getting ready to spawn.
Check for white spots. Male goldfish develop white spots on their gills called breeding tubercles when they are ready to spawn.
How to Determine If Goldfish Are Male or Female
When getting a goldfish as a pet, determining if it is male or female may be a priority. Whether it is just in order to decide what to name the fish or if you are looking to get a pair so that they can breed, determining if a fish is male or female is not as easy as with other pets, but it can still be done.