A very frequent question asked by the hobbyist when starting out with breeding their aquarium fish is, "How do I discover the sex of my fish"? Of course the answer depends very much on the particular species. With certain species there are quite obvious differences between the sexes but unfortunately with some species there are no apparent differences at all.

Let's start with the easier to determine varieties of popular species. Betta fish are an obvious example. Males have all the looks and beauty, with long flowing finnage and bright coloration, while females are the "plain Janes" with much more subdued colors and shorter fins. The females also have a pimple-like white bump below their vent.

Livebearers are generally easy to sex. In guppies, swordtails, mollies and their relatives, sexually mature males have a modified anal fin, the gonopodium which is used for transmitting sperm into the female. In addition, male guppies are smaller and more colorful than females, male swordtails have long, pointed projections on their tails and female mollies are plumper than the males.

In the case of the egg-layers the sexes are far more difficult to tell apart. The length of the fins, particularly the anal and dorsal fin, are often longer in males in some species of cichlids and gouramis. Young fish will not have fully developed fins and therefore cannot be differentiated. Mature female egg layers also have larger bellies than males because of their enlarged ovaries and can account for up to fifty percent of the total body weight of the fish.

There are also exceptions to these general differences and usually breeders overcome the problem of sorting males and females by buying a group of youngsters and letting them grow up and mature together, thus having a chance of getting a mix of each sex. They can then be allowed to pair off naturally.

A process known as venting is used by serious and experienced breeders to distinguish the sexes. The vent is an opening between the anus and the anal fin, where the fish can expel either sperm in the male, or eggs in the female. The process involves removing the fish from the aquarium and handling it to examine the vent. Knowing exactly what to look for and carrying out the procedure without harming the fish is really a process which can take a lot of practice before it can be done accurately.

So it has to be said that trying to determine the sex of a fish is really not an "exact science" and the easiest way to ensure that you have a good breeding pair is to raise a group of at least half a dozen juveniles together. This way you can then be assured of having some of each sex ready to go ahead and happily reproduce the species when they are mature.

Jen Clements is an experienced fish breeder and aquarium specialist. She has a website devoted to providing information and hints for hobbyists. Jen has a FREE e-course "Aquarium Fish Breeding Secrets" that you can get right now at Aquarium Fish Breeding