Gender in Koi.

Male koi and goldfish are more streamlined. Female Koi are usually shorter bodied and "full" through the shoulder area, especially just behind the gills.

Males in the springtime have breeding tubercles, which are (almost imperceptible) white spots on the gill covers and the leading ray of the pectoral fins. Females lack these tubercles.People are surprised that these breeding tubercles can be seen in Goldfish of three inch length and Koi of only six inch length in some cases.

These breeding tubercles are sometimes mistaken for "white spot disease" and harshly treated.