The difference between male and female goldfish is that male goldfish have bigger and thicker pectoral fins, a smaller anal opening and small bumps on the fins and gills when they are interested in mating. Female goldfish will not get the small bumps during mating season and they have a larger anal opening and smaller pectoral fins.
Another way to tell the difference is to see which fish fatten out around the bottom. These fish are usually female and this indicates that mating season has arrived. It is incredibly difficult to tell the difference between male and female goldfish when the fish are young so it is recommended to start looking at sex when the fish are at least 1 year old, according to Fish Channel.