You will know if your goldfish is a boy or a girl by evaluating the following attributes:
- Size – female goldfish are generally larger than male goldfish.
- Body shape – female goldfish have a thicker and rounder belly, while males have a thinner, more streamlined body. During spawning season, a female goldfish’s belly will also appear somewhat asymmetrical or lopsided.
- Vent (anal opening) – The vent of a female goldfish tends to be rounder and protrude out, especially as the breeding season approaches.
- Anal fin – a male goldfish’s anal fin is slightly thinner than the female’s.
- Breeding tubercles – tubercles are small white spots on the gill shields. Only male goldfish have tubercles. In younger males, tubercles only appear during the breeding season, but in older males, tubercles may be present all year long.
- Behavior – Male goldfish will start to chase female goldfish around as breeding season approaches.