At first glance, male and female goldfish look the same. However, there are some clues to gender. The easiest way to recognise the sex is during the mating season, which can begin from the age of four months in goldfish.

 

Characteristics of male goldfish

During the mating season, male goldfish develop white nodules on the gills, which are often mistaken for a typical goldfish disease. These nodules or spots can spread to the pectoral fins.
The pectoral fins of male goldfish are larger and longer than those of females.
The stomach is smaller and firmer.
The anus of the male goldfish is smaller and more oval-shaped.
The body shape of a male is more streamlined than that of a female.
Males have fine grooves on the underside that run from the ventral fins to the gill opening. In females, these grooves are less pronounced or not present at all.
The behaviour of the goldfish makes it easy to tell who the male is, as the male usually chases after the female, nudging her backside.

Characteristics of female goldfish

In general, female goldfish are larger and plumper than males. Especially during the breeding season, the female's belly enlarges due to the volume of the eggs.
The belly of a female goldfish is softer.
The pectoral fins are shorter and rounder than those of the male.
The anal opening is rounder and larger.