Many fish owners are interested in learning the sex of their pet fish. In some cases this is an easy thing to determine and in other cases it is quite difficult, if not impossible. The reason is the wide variability between species of fish. And with hundreds of species kept as pets, one can easily imagine that what works with one species won't in another.

Sexual dimorphism is a term used to describe a species of animal where the males and females differ physically to the naked eye. Sexually dimorphic species include many of the livebearers, the majority of cichlids, and goldfish at certain times of the year. Generally speaking, species that have courtship as part of mating are dimorphic, and species that spawn in large groups tend not to be sexually dimorphic. Another factor to consider is the age of the fish. Many dimorphic species don't develop their differences until they are sexually mature, meaning that all juveniles of some species may look exactly alike.
        
The differences between the sexes of dimorphic species may be subtle or quite remarkable. Livebearers belonging to the family Poeciliidae (mollies, guppies, swordtails, platies, and wags) are easy to distinguish. The male fish possess a gonopodium, which is a modified anal fin used for copulation. The female fish's anal fin is shaped like a triangle. Sexually mature males of many New World cichlids have a large hump on the head. In some cichlid species, the male's dorsal and anal fins are much longer and larger than the female's. When spring arrives, the male goldfish develops numerous small bumps on the head, opercula (gill plates), and pectoral fins. These bumps are called nuptial tubercles and usually disappear at the end of the breeding season. They are occasionally mistaken for external parasites.

The sex of some species, like the koi, are difficult to determine unless the fish are ready to spawn. In a koi pond, the females will generally be larger and more full-bodied than the males. In addition, a female ripe with eggs will have a swollen abdomen which will set her apart from the more streamlined males.

Regardless of species, the best thing you can do to determine the sex of your fish is to obtain a book or magazine that discusses the natural history of your fish. Using this approach, you should be able to learn about any physical or behavioral differences between the sexes of a particular species. Your local pet store clerk can also be an excellent resource for providing tips on how to tell a male from a female fish.