swordtails

Swordtails are small freshwater fish in the family Poeciliidae, commonly kept in home aquariums. The term usually refers to the swordtail Xiphophorus hellerii, though many color strains and hybrids exist in the hobby. Males typically have a long, sword-like extension on the lower lobe of the tail fin, whereas females lack this feature and are usually larger.

Native to the rivers, streams, and swamps of southern Mexico and northern Central America, swordtails prefer warm, moderately hard water with a neutral to slightly alkaline pH. In aquariums they tolerate a range of conditions but thrive with stable temperatures around 22–28°C, pH 7.0–8.0, and good water quality.

Swordtails are livebearers. Females give birth to free-swimming young after a gestation period of roughly 28 days. Broods can range from a few to several dozen fry, and females may store sperm and produce multiple broods from a single mating. Males use a gonopodium, a modified anal fin, to transfer sperm.

They are omnivorous and will accept most flake foods, pellets, and live or frozen foods. In the aquarium, keep them in groups with at least one male for several females; provide plenty of plants or hiding places. They are generally peaceful and compatible with many community fish but may eat smaller tankmates and may harass slower fish if stressed.

Many color morphs and patterns have been developed through selective breeding. Swordtails are prolific breeders in home aquariums, and new hobbyists should consider separating fry or using a separate breeding tank to control population growth.