"How can you tell whether your Goldfish is male or female? You might want to know because you want to breed them, or give them the right name before Jack turns out to be the mother Goldfish! Here are some clues that might help you sort out the Goldfish boys from the Goldfish girls:
* The female Goldfish are slightly bigger than the males and look even heavier in the abdomen when they are full of eggs.
* Male Goldfish develop white spots on their gills and pectoral called “breeding tubercles” during spawning time.
* Males have midline ridges on their undersides beginning from the back of their pelvic fins and ending at their vent opening. This feature can be absent or smaller in females.
* Males have firm abdomen while in females; the area between the pelvic fins and the anal fins is more pliable.
* Male Goldfish have longer and pointed pectorals as well as stiff fin ray, while females have more rounded pectorals and shorter, finer front fin rays.
* Male goldfish have smaller and more oval anal openings and the anal fins are not as thick as that of the female goldfish who has a larger and rounder anal opening with a slight protrude. The anal fin is also thicker.
* The spawning time is the easiest time to distinguish the boys from the girls. The female opening will look larger and more swollen as the eggs begin to ripen. She would look big and heavier. Since fish reproduce through external fertilization, you can make out when a male is ready by the white breeding tubercles and you can actually push out the milt through his ventral opening by running a gentle finger along his sides.
* There is one more obvious, but kind of time taking way to make out which one of your fish is male and female. Find out who is chasing who – the chaser will be the male and the chased and tired out one will be the female! It’s the natural tendency on part of the male goldfish to chase and nudge at a female’s rear part."
I hope that helped out =)