How to differentiate between male and female goldfish?
Goldfish are difficult to sex until they are mature. At maturity, especially when spawning behaviour begins, it is easier to tell males from females. The male will develop bumps on his gill plate and possibly the first ray of his pectoral fins. These are called breeding stars or breeding tubercles. The male will be more slender than the female which will be more rounded when she fills with eggs. The shape of the vent, where sperm or eggs are released from, is different on males and females. On the male it is concave, pressed in, like an “inny” belly-button on the male and convex, pushed out, like an “outty” belly-button on the female. The easiest way to sex goldfish is during spawning. The one expelling eggs is the female and the one fertilizing the eggs is the male.
