Fish

Swordtail

Xiphophorus hellerii · also called swordtail, green swordtail

Least Concern → omnivore Rivers & wetlands Rainforest

A swordtail is one of the standard first fish of the aquarium hobby, a hardy, active livebearer that breeds without any help and thrives in plain tap water. Its name comes from the long coloured blade trailing off the bottom of the male’s tail — an ornament that female swordtails prefer, and that has made the fish a favourite subject for studying how such ornaments evolve.

Overview

The swordtail, Xiphophorus hellerii, is a livebearing fish of warm, flowing, weedy streams on the Atlantic side of Mexico and northern Central America. It grazes algae and picks small invertebrates, and the female gives live birth to broods of 20 to 100 every few weeks, storing sperm for later broods.

The sword

The sword is a long extension of the lower tail-fin rays, often as long as the male’s body, and it grows only in mature males. Females prefer longer swords — so strongly that they will court swordless relatives given a fake sword, which suggests the preference came first and drove the ornament’s evolution. Some females change sex late in life or when no males are present, growing a sword and breeding as males.

Platies, hybrids, and cancer

Swordtails interbreed freely with platies, their close cousins, and most pet swordtails are hybrids carrying platy colour genes. Certain swordtail–platy crosses develop melanoma from runaway pigment cells — a finding from the 1920s that made Xiphophorus one of the oldest animal models in cancer genetics.

Status

Least Concern — the green swordtail is common and widely introduced, sometimes invasively. Several small-range Xiphophorus confined to single Mexican rivers are Endangered or Critically Endangered.

Swordtail: common questions

What is the difference between a swordtail and a platy?

They are close relatives in the same genus, Xiphophorus, and interbreed so freely that many aquarium fish are hybrids of the two. The clearest difference is body shape and the sword. A swordtail has a longer, more torpedo-shaped body, and the mature male has the trailing sword on his tail. A platy is shorter, deeper-bodied, and stockier, and no platy has a true sword. Swordtails also grow larger, up to about 14 cm including the sword, against roughly 6 cm for a platy, and are a little more active and inclined to jump. Both want the same hard, neutral to alkaline water and the same care.

Do I need a male and female swordtail, or will they breed anyway?

If you have both sexes, they will breed, continuously and without any help, because they are livebearers and a mated female stores sperm for months. A common problem is buying a group that turns out to be all female, only for one to change sex and start a population. If you do not want fry, keep a single-sex group, ideally all males, though males can harass each other, so keep either one male or a group of five or more to spread the aggression. In a mixed tank, keep two or three females per male so no one female is pestered constantly.

Why do some female swordtails turn into males?

Swordtails have a flexible sex-determination system, and in some individuals a fish that has functioned as a female for months or years will develop male features, including a sword and the modified anal fin used to transfer sperm, and begin behaving and breeding as a male. It happens most often in older females and in groups with no males present. Whether these late-changing males can actually father young varies. It is a genuine sex change, not just a cosmetic one, and it is one reason a swordtail tank can produce surprises.

Are swordtails good fish for beginners?

Yes, they are a classic beginner livebearer, hardy, active, colourful, inexpensive, and happy in ordinary hard tap water without special treatment. The main things to get right are tank size and a lid, because swordtails are strong swimmers that need a tank of at least 75 to 115 litres and are notorious jumpers, and sex ratio, to avoid either a fry explosion or males fighting. They are peaceful with other community fish of similar size, though large males may nip long-finned tankmates.

Sources & further reading

Written by HideShark Editorial Team · reviewed by HideShark Science Desk. Published September 11, 2026. Last updated September 11, 2026.

Quick facts

Scientific name: Xiphophorus hellerii
Diet: An omnivore that grazes algae and biofilm and picks small invertebrates, insect larvae, crustaceans, and worms from plants and the bottom, and in the aquarium takes flake, pellet, vegetable foods, and live or frozen foods such as bloodworm and brine shrimp
The sword: The sword is a long extension of the lower rays of the male's tail fin, often as long as his body, coloured yellow, orange, or green and edged in black, and it grows only in mature males, develops later than the rest of the fin, and is used in courtship and in sizing up rivals
Females prefer a longer sword: Female swordtails prefer males with longer swords, and will even court males of swordless related species onto which an artificial sword has been added, evidence that the female preference existed before the sword evolved and helped drive its origin
A livebearer: Swordtails bear live young rather than laying eggs, the female producing broods of 20 to 100 every four to six weeks and able to store sperm for several later broods, and the fry are large and immediately independent, though the adults will eat them
Females that become males: Some female swordtails can change sex, developing a sword and male behaviour and function, usually late in life or when no males are present, so that a single-sex group of females may end up with a working male
Blurred with the platy: The swordtail interbreeds freely with the platy, its close relative in the same genus, and much of the huge range of aquarium colours, red, black, tuxedo, wagtail, pineapple, and lyretail, comes from crossing the two, so most pet swordtails are hybrids rather than pure Xiphophorus hellerii
A century-old cancer model: Crosses between swordtails and platies can produce hybrids that develop melanoma from oversized pigment cells, a discovery from the 1920s that made the genus Xiphophorus one of the first and longest-running animal models for the genetics of cancer
Range: The Atlantic slope of Mexico and northern Central America, from Veracruz through Belize, Guatemala, and Honduras, in warm, flowing, well-vegetated fresh water including streams, rivers, springs, canals, and ponds from the lowlands into the foothills, plus established feral populations from aquarium releases in warm regions around the world.
Conservation status: Least Concern → ≈ The green swordtail is common across its native range and widely introduced elsewhere, and is assessed as Least Concern, though wild populations can be affected locally by pollution, habitat change, and interbreeding with escaped aquarium fish. Feral swordtails from aquarium releases have established in warm waters on several continents and are considered invasive in places such as Hawaii and parts of Africa and Australia. Several other members of the genus Xiphophorus have small ranges in single Mexican river systems and are threatened, with the Monterrey platyfish and a few swordtail species listed as Endangered or Critically Endangered through water extraction, pollution, and introduced fish in the wild