Unlike most fish species, livebearers give birth to free-swimming fry instead of laying eggs. In live-bearing species, the eggs get fertilized inside the female, just like in mammals and birds. The male fishes have therefore developed some form of reproductive organ that can be used to deliver sperm. The exact configuration will vary from species to species, but having a fin transformed into a so called gonopodium is very common. The male fish will use the gonopodium to reach inside the female and fertilize the eggs. Another similar solution is found on the males from the families Goodeidae and Hemirhamphidae – the andropodium. An andropodium will always fold when being inserted into the female fish, while a gonopodium flip forward instead.
Breeding livebearers in the aquariums is usually quite easy, since the fry will not be born till they grow large enough to fend for themselves – at least to some extent. Many of the most commonly kept beginner species are livebearers, such as Guppy, Molly, Platy and Swordtail. All these species belong to the family Poecilidae, but there are livebearers to be found in many other families as well. Guppy, Molly, Platy and Swordtail are known to easily spawn in captivity without needing any coaxing or pampering from the aquarist. You should however be aware that adult fish happily eat small fry.
In some species, the female fish can tore the sperm from one mating for long periods of time. If your livebearers breed in an aquarium where no males are present, it is because they have been fertilized earlier and stored the sperm until now.
Most livebearers are schooling and should be kept in groups, but there are exceptions to this rule so researching your particular species is always a good idea. A harem, where one male is kept with several females, is a good choice if the male is aggressive toward members of his own sex or towards females. The more females, the less aggression towards each one of them.