Instructions
Get someone else to help you look closely at your fish. It isn't easy to keep up with the movement of your fish, especially when you are trying to concentrate on one small area of your fish's body.
Look for a thick pectoral, or breast, fin. The male goldfish will have a thicker pectoral fin than a female.
Look for small white specks or bumps on the gills of your goldfish. The males will have these specks, or tubercules, on their gills while the females won't.
Female goldfish have a much smaller pectoral fin on their bodies. This fin is also much thinner than the male's pectoral fin.
If you don't see the white tubercules on your fish's gills, she's a female. Please look carefully for the white specks. You don't want to miss them and think your male goldfish is a female.
How to Tell the Gender of Fish
Many people have aquariums in their homes, filled with goldfish. Often, they won't have a clue that they have fish of both genders in their aquariums until they see baby fish swimming around. It takes a little patience, fast eyes and detective work to figure out the gender of all your fish.