Males often develop small white breeding tubercles on the gill covers and pectoral fins during breeding season
Males are usually slimmer and more streamlined
Females are often rounder and fuller-bodied, especially when carrying eggs
Females may have a slightly protruding vent, while males usually have a flatter one
During breeding season, males may chase females more actively
Males may show a more pointed pectoral fin shape, while females often have rounder pectoral fins
The easiest way to tell is to compare several goldfish of the same age and species
Sexing young goldfish can be difficult because differences are often not obvious
A swollen abdomen in a healthy fish can indicate a female with eggs
The most reliable signs usually appear when the fish is mature and in breeding condition