The popular and easy to care for molly, platy, guppy, and swordtail all belong to a family collectively known as live-bearing tooth-carps, or Poeciliidae. These fish closely resemble killifish and minnows. They are all natives of the tropical and sub-tropical regions of the western hemisphere, from Central to South America, where they live in small streams. These are peaceful fish, which make them suitable for the community aquarium. None of them grow much bigger in the wild than 6 inches and in the aquarium they barely grow to 4 inches.
What distinguishes these fish is their method of breeding. Rather than lay eggs that the male then fertilizes, these fish hold their eggs internally. The male fertilizes them with a specialized fin called a gonopodium, and the young hatch within the female and then swim free. Females can produce several hatches of young from a single fertilization.
The two genera that make up most of the aquarium species are Poecilia, which include the guppies and mollies; and Xiphophorus, which includes the platys and swordtails. While the wild species are colorful enough (Poecilia means many-colored), hobbyists and breeders have taken advantage of their easy breeding habits to create dozens of vibrantly colored varieties.
Swordtails and platys have been interbred so much that sometimes there is little to distinguish one species from another, except the elongated sword-shaped fin of the male swordtail. The major color variations of platys are blue, red, red wag (which means it has a black tail), sunset and gold twinbar. Swordtails have the same color varieties.
Of all the Poeciliids, the guppies are probably the most popular due to the rainbow of colorations in this small, 2-inch fish, as well as the exotic variations of its banner-like tailfin. The common names derive from the shape of that tailfin: flagtail, veiltail, fantail, and triangletail, among others. By contrast, mollies display only a few varieties of tailfins and fewer color varieties, mainly black, green, gold and silver.
They like to school, so it's best to keep a single male with two or three females. They require temperatures around 75 degrees Fahrenheit, somewhat hard water and a natural to slightly alkaline pH. Mollies do best when a teaspoon of salt is added with every 2 1/2 gallons of aquarium water.
These are also easy fish to feed. They will readily take fish flakes and do well without live food, although they will eat brine shrimp. Since the young are already fully formed when released by the female, they can be fed very finely crushed flake food.
While these are fairly hardy fish, overcrowding or infrequent changes of water can make them susceptible to disease. Remember that although these fish breed readily, breeding and bearing young puts any fish under a great deal of stress so they are best isolated when giving birth and after. With hatches coming at intervals of some 30 days, the real concern can quickly become population control (although if the fish give birth in the community tank, other fish and ever the parents, will snap up the young for food).