Swordtail

About the species

The swordtail (Xiphophorus hellerii) is the largest of the three livebearers in this group – males, with their signature extended lower tail-fin lobe (the 'sword'), reach up to 14 cm, so they need a noticeably bigger tank than the related platy. This popular omnivorous fish from southeastern Mexico and Central America is generally peaceful, but males establish a clear hierarchy, which surprises many owners who expect an otherwise 'easy' livebearer.

A tank that needs to be long, not just big

A footprint of at least 120 × 30 cm, roughly 100 litres, is recommended – considerably more than a platy needs (54 l), and more than mollies too. Care references rate swordtails as moderately demanding, partly because of this space requirement and the social dynamics among males.

Hierarchy among males

In too small a tank, males establish a dominance relationship and spend considerable energy maintaining it – this isn't aggression toward other species but within-species competition for rank. The fix is either enough space or keeping a single male with a group of females, rather than several males in a cramped tank.

Water

A temperature of 20–27 °C, pH of 7.0–8.0 and general hardness of 10–25 °dH suit this species; carbonate hardness of roughly 6–15 °dH helps keep pH stable. Swordtails tolerate a wider temperature range than mollies but, like mollies, do better in hard, alkaline water.

Décor

In the wild it inhabits a broad range of habitats, from fast, rocky streams to springs, ditches, ponds and rivers, so in the aquarium it does well both in a densely planted setup with a dark substrate and in a stream-style layout with water-worn rocks. Floating plants and roots that diffuse the light are appreciated too.

Telling males from females

Adult males are usually smaller than females and have a gonopodium along with the characteristic extended lower tail-fin lobe; in some bred forms the upper tail lobe or other fins may also be extended, occasionally even in females.

A range that extends well beyond its native waters

The swordtail is native to southeastern Mexico, Guatemala, Belize and northwestern Honduras, but its popularity in the aquarium hobby has led to it being introduced into waterways on every continent except Antarctica. Where it occurs in large numbers outside its native range, it can negatively affect native species and ecosystems – one more reason it should never be released into the wild.

When a female 'becomes' a male

Swordtails show a documented phenomenon where some adult females develop male characteristics, including a gonopodium – this is hormonally driven, not an actual change of reproductive sex, and it's relatively well studied in this species compared with other livebearers.

Crossing with the platy

Swordtails and platies (Xiphophorus maculatus) interbreed readily in the aquarium, so keepers who care about colour or fin-shape purity generally don't keep them together. Most commercially sold 'swordtails' already carry some hybrid ancestry anyway.

Diet

An omnivore that feeds on invertebrates, algae and plant matter along the banks in the wild; in the aquarium it accepts almost any quality food, with a regular plant-based component being beneficial.

Breeding

Like other livebearers, it gives birth to fully developed fry. A higher ratio of females to males is recommended to reduce harassment, and it makes sense to move a gravid female to a separate tank before she gives birth.

Pros & cons