Knowing the sex of goldfish is not only important for breeding purposes but also in the naming of your fish. It is harder to distinguish between male and female goldfish when they are young; however, as they mature, you will be able to notice certain differences that are more characteristic of one or the other. Until you are certain of the sex, it may be best to choose a generic name.
When looking down on top of the body of a goldfish, the mature female's body is generally more plump and more round compared to that of the male goldfish. The male goldfish will be longer and thinner in comparison to the female. The sides of a female's body will also appear to be uneven; one side will stick out more than the other.
Tubercles are little white spots or bumps that develop on mature male goldfish during spawning. The male will develop these little bumps on their heads and on their gill covers, called operculum. These spots will also be noticeable on the leading edge or ray of the male's pectoral fins during breeding.
With male goldfish, the first ray of the pectoral fin is thicker and more stiff in comparison to the female's. The pectoral fin is located just under and behind the gills. The female's fin is rounded, and the male's is longer and more pointed. On the female, the leading edge of the anal fin, located behind the vent, will be thicker.
The vent is the anal opening of the goldfish. When the female is ready to breed the shape of the vent will change. This area will become larger, round and stick out; it will be convex in appearance. The male's vent is smaller, thinner and concave or indented in appearance.
If you observe the fish during spawning, you will see that the male goldfish is the more aggressive of the two. He is the one doing the chasing. He will bump the female to try and get her to release her eggs. The female is the one being harassed and the one being chased.