Re: how can we the tell gender of a goldfish?

Many fish are hermaphrodites -- which means that any one fish can make both 
sperm and eggs -- but goldfish are either male or female, just like people. 
Unfortunately, most of the time you can't tell the difference between the 
two sexes. The only time you can tell whether a goldfish is male or female 
is during their breeding season. This is usually in the spring, since 
goldfish breeding behavior is triggered when the water they live in gets 
warmer. When goldfish get ready to breed, the females become fatter as they 
fill with eggs to spawn, and the males develop tiny white spots on their 
head and gill covers. These breeding tubercles can be seen on the black 
fish in this  picture 
.


Refs: Ostrow, M. 1985. Goldfish: Everything about aquariums, varieties, 
care, nutrition, diseases, and breeding. Barron's: Woodbury, New York.