Livebearers are fish that give birth to live, free-swimming babies instead of laying eggs. Common livebearing fish for tropical freshwater aquariums include Swordtails, Platies, Variatus, Mollies, and Guppies. These bright and peaceful little fish come in a variety of colors and patterns and can easily be placed in a tropical community. While these fish share many similarities in behavior and care needs, there are subtle differences that set them apart.
Swordtail (Xiphophorus helleri) males are easily identified by the elongated lower lobe of their tail fin. Males may show aggression towards each other in the aquarium. Platies (X. maculatus) and Variatus (X. variatus) have a slightly more stocky build than swordtails. Otherwise they are very similar and may even interbreed. Mollies (Poecilia velifera, P. latipinna, P. sphenops) are even more robust and have the ability to adapt to a wide range of salinity; they can live in pure freshwater or ocean water. Males have a much larger dorsal fin than females that they flare when courting or showing aggression. Guppies (P. reticulata), particularly males, are easily recognized by their huge tails and amazing coloration though they can be much more delicate, especially during transport and acclimation. Many other livebearer species exist, but they are not as frequently offered for aquariums.
Livebearers are best-known for their prolific breeding. Livebearers have the ability to deliver several batches of young from a single fertilization from a male. Gestation ranges from 4-6 weeks for most to 8-10 weeks for larger species like Sailfin Mollies. Each batch may number from 25 to more than 100. Most Livebearer parents are cannibalistic, and will eat their babies. To collect or protect the babies, place pregnant females into a breeder net or trap until she delivers, or provide plenty of plants and ornaments where the new babies can find safe haven to grow. In a heavily planted tank at least some of the young usually survive.
Keep livebearers in a well-maintained aquarium with live or artificial plants. They prefer a little aquarium salt, though it is not necessary. These fish are safe and peaceful in community tanks. Males have a modified pointed anal fin known as a "gonopodium" while this fin is held fanned out in females; a "harem ratio" (one male to two or more females) is generally recommended to reduce aggression and harassment and for the more successful breeding broods. They will feed on any commercial flake or pellet food, and may also be fed small freeze-dried or frozen foods as treats periodically. They will also eat some algae as it appears in the aquarium.