Goldfish with Salt dusting on their pectoral fins and gill covers
It’s possible to gender (tell the sex of…the male, or femality) of Goldfish and Koi as small as a few inches, especially in the Springtime. The way to do this is observe closely the gill covers, and the leading edge of the pectoral fins. (Those are the fins that emerge immediately behind the gill opercula).
The gill covers and the leading edge of the pectorals will feature a row of almost imperceptible, small white bumps. Except for their orderly array, they could be mistaken for ICK. (Ichthyophthirius multifilis.)
In larger fish, those over 6 inches for Goldfish and those over 10 inches for Koi, these bumps are pretty easy to see. In smaller fish, the bumps would be absent in the cooler months and late summer, but can be palpable with the tip of the tongue if your fingertips are not sensitive enough to be sure about.
Remember, in Springtime, these bumps are more prominent.
Finally, the MALES will bear these spawning tubercles or pimples. The female will have no such bumps or roughness of the pectoral leading ray.
Goldfish With Bumps on Pectoral Fins and Gill Covers
So, MALES are rough and bumpy, and
FEMALES are soft and smooth! Simple to remember.