Goldfish start maturing at 18 to 24 months of age, or when they grow to at least 4 - 5 inches long. As the breeding season approaches - March thru June - the male goldfish develops tiny white tubercles on the operculum, the hard, bony plate covering the gill, and the front or bony edge of the pectoral fin.
These tubercles are visible and feel rough to the touch. They are called "salt spots" by the old timers, since the white tubercles resemble coarse grains of salt.
Normally, the pectoral fin on the male goldfish is long and slender, while that of the female is short and more rounded. Also, the bodies of the female goldfish are more rounded because of the developing egg sacs.
If is difficult to determine sex of juvenile goldfish or if the goldfish is small in size.