Goldfish become sexually mature when they reach 3 to 5 inches or so. In a pond setting, this usually occurs after about a year. Fancier goldfish, since they grow slower and to a smaller size than regular goldfish, sexually mature at a smaller size/length. The exact age of maturity varies with the variety of goldfish and its living situation and can occur anywhere from 9 months to 2 years after hatching.
Some people will say that sexing goldfish is only possible during breeding. During this time, males chase females who lay eggs. Also, males develop white spots or pimples on their gill covers called breeding tubercles. Often, people mistakenly believe these white spots to be the parasite white spot or ick and treat the healthy fish. Females may develop a tubercle or two but not nearly as many as a mature male. Even during non-breeding times, an experienced person can sex adult goldfish. Koi can be sexed similarly to goldfish but not as readily.
Mature female goldfish are usually (but not always) larger, fuller, and have a convex (sticks out) anal opening (vent). They often have a thicker leading ray of the anal fin. Mature males are usually (but not always) thinner, longer, and have a concave anal opening. Excited males may retain breeding tubercles year-round on their gills and the leading ray of their pectoral fins. The leading rays of the seasoned mature males pectoral fins are often much thicker than females fins and covered with tubercles. The mature males pectoral fins may also be longer and more pointed than females pectorals. The breeding tubercles on the gill plates and pectoral fins are especially obvious on a Black Moor since the spots are white on black(good example).
