Livebearing Fish(from About.com)

Of all known species of fish only about 1% of all fish are viviparous, or livebearing. Yet despite their relatively low numbers, the livebearing species have been a staple in the aquarium industry since it's beginning.

Livebearers include some of the most common as well as uncommon fish found in the trade today. Anableps, Goodeides, Guppies, Halfbeaks, Mollies, Mosquito Fish, Platies, and Swordtails, are some of the richly varied livebearing species available to fishkeepers. On the other end of the spectrum is one of the most unusual and ancient fish species in existence today. The Coelacanth ("see-la-kanth"), once thought extinct for four hundred million years, has been found very much alive off the coast of Africa.

Whether ancient or common, all share a common trait - they bear well developed live young. The fact that they bear live young isn't the only reason aquarium enthusiasts find them so desirable. They are also prized for their hardiness, their ability to adapt to a variety of habitats, and for their attractive coloration.

Habitat
Although requirements vary somewhat by species, in general livebearers favor moderately hard, slightly alkaline water (pH of slightly above 7). Because tap water usually fits those parameters, it is relatively easy for most fish owners to provide a suitable habitat for them. They also tolerate a fairly wide range of temperatures, from the upper sixties to the low eighties.

The addition of a tablespoon of aquarium salt per five gallons of water is recommended for most livebearing species. Although peaceful, livebearers should only be included in community tanks if kept with fish that thrive in similar water conditions.

Diet
Livebearers are easy to feed, readily accepting everything from flake to live foods. Because they tend to be grazers, frequent small feedings are preferable to infrequent large feedings. Vegetable matter is a cornerstone in the diet of most livebearers, particularly for mollies. Spirulina added to the diet will help keep them from eating any live plants you have in the aquarium.

Breeding
As their name aptly expresses, livebearers give birth to live young rather than laying eggs. They are easily bred, requiring very few special accommodations. Keeping several females for each male is advisable, as persistent courtship by the male can be stressful if there is only a single female.

It is easy to determine the sex of most livebearers by noting the differences in the anal fin. The female has a fan shaped anal fin, while the male has a rod shaped anal fin (see photos above in the right hand column). The modified anal fin in the male is known as the gonopodium, and is used to inseminate the female. Females are able to store sperm cells from the male, and can produce several broods of young from a single insemination. The gestational period is about four weeks for most livebearing species.

If the adults are hungry, and adequate hiding places are not available for the young, they will consume the fry. Providing plenty of plants, particularly dense floating plants, will ensure that at least some of the fry will survive to adulthood. Artemia, commercial fry foods or finely ground flake foods are all suitable for feeding the fry.