Goldfish is a fish very famous and being kept by most peoples. its species name is carassius auratus. since there is many of goldfish lovers want to breed them, i will show you how easy to determine male from female. so you have another easyness in selecting the parents.
Step 1: Male and Female 1
Male has slimmer body than female. while female more rounder when we compare them from the top.
on the right side of the picture is female. and the left side is male.
Step 2: Male and Female 2
another easy to determine goldfish's sex is the male has many white dots near the head, especially at the gill, and pectoral fins. as u can seen in the picture.
thanx and good luck.