Green swordtail

The green swordtail (Xiphophorus hellerii) is one of the world's most recognisable and widely kept aquarium fish, native to freshwater systems from southeastern Mexico through Guatemala, Belize, and into northwestern Honduras. Males develop the characteristic elongated lower caudal-fin lobe or 'sword' that gives the species its common name, reaching up to 14 cm in total length, while females are larger and rounder-bodied at up to 16 cm. Wild fish display an olive-green to golden body with a red-brown lateral stripe, but decades of selective breeding have produced an extraordinary variety of ornamental colour morphs including red, black, wagtail, tuxedo, marigold, and albino forms — many of which result from hybridisation with other Xiphophorus species such as X. maculatus and X. variatus.

Swordtails are generally peaceful and mix well with similarly-sized community fish, though in confined spaces males form dominance hierarchies and may spend significant time maintaining their positions. They are undemanding and adapt readily to a range of water conditions, which along with their prolific breeding has contributed to their establishment as feral populations on every major continent except Antarctica. In nature they are generalised omnivores feeding on aquatic and terrestrial invertebrates, organic detritus, algae, and plant material. In the aquarium they accept almost anything offered, but a balanced diet of quality dried products supplemented with live and frozen foods such as Daphnia, Artemia, and bloodworm will bring out their best condition.

An aquarium with base dimensions of at least 120 × 30 cm (roughly 108 litres) is the minimum for a small group. The tank benefits from a dark substrate and planted areas with open swimming space, plus some floating plants to diffuse overhead light. Wild forms appreciate a set-up arranged like a flowing stream with water-worn rocks. Recommended water parameters include a temperature of 16–28°C, pH 7.0–8.0, and hardness 179–447 ppm. Females are livebearers capable of storing sperm for months and producing broods regularly; fry are relatively large at birth and accept crushed flake and baby brine shrimp from the start.