Livebearers are members of the group called cyprinodonts. There are many species of livebearing fish (fish that give birth to live young rather than laying eggs), but in the fishkeeping hobby, the term “livebearer” usually refers to guppies, mollies, platies, and swordtails.
In the past, guppies were extremely popular fish among beginner hobbyists. They are still commonly kept today, but more often by experienced hobbyists. Guppies are an especially popular fish to breed, particularly fancy guppies.
Although mollies are widely available from pet stores and other sources, they are not a good choice for novice aquarists (see Not Recommended).
Platies are highly recommended as a great fish for the first-time hobbyist. They are widely available, hardy, peaceful fish that come in a great variety of color combinations. Most wild platies are silver, blue, or orange, while domesticated platies can be white, yellow, gold, orange, red, blue, or black. In a community tank, platies will school. A smaller tank can hold up to eight pairs of platies, while a larger tank could house up to 15 pairs!
Cousins of platies, swordtails make great aquarium residents. Their name comes from the sword-like extension the males have on their tail fin, which they use to attract females and to intimidate other males. Wild swordtails are generally green with iridescent or black markings, while domesticated swordtails are available in a wide variety of colors. Swordtails are larger than platies and the males can be quite aggressive, so keep them in a fairly large tank, and keep only one male with several females.