Xiphophorus hellerii (swordtail)

Xiphophorus helleri which is also known as green swordtail belongs to the poeciliid family. The species were also known as ”Mollienisia helleri”.
The species is native of Central America (Mexico to northwestern Venduras) but, but occurs as introduced in different countries presumably due to the aquarium hobby. In general, the species occur in lowland backwaters (lakes, swamps) and highland waters (lakes, rivers) with cooler temperature from other species of the family (mollies). The water in these habitats is slightly hard and alkaline.
Many of the domesticated swordtail varieties (green tuxedo, albino, red wag, etc.) were developed by selective breeding the wild form of swordtail and hybridizing with other species of Xiphophorus. The natural form usually has a greenish, blue, red coloration. In general swordtails are hardy fish, but some of the domesticated varieties are not as hardy as the wild form or the red forms of swordtails. The species have an average of 3 years lifespan, yet if cared properly can live up to 5 years.
The species have sexual dimorphism with the males being smaller than females. The male swordtails, like the rest of the Poeciliidae family, have their anal fins modified to the rod-shaped male’s reproductive organ, the gonopodium. Furthermore, male swordtails possess a sword-like extension to the lower rays of the caudal fin. However, females sometimes develop the sword-like extension as a result of hormonal changes. These individuals are sterile as males. Swordtails have an average size of 4 inches (10 cm).
Like the rest of the poeciliid family, this peaceful, social fish is an excellent choice for beginners because of their hardiness and adaptability, easy to reproduce and with a variety of colorations to choose. They like to be kept in groups, with the recommended ratio male to female to 1: 3, as female swordtails get stressed from the persistent chasing by the male swordtails throughout reproduction. Some compatible species with swordtails are other species of the Poeciliidae family like guppies and platies (If you don’t want hybridization, don’t put them together), rasboras, bottom dwellers like Ancistrus, etc.
Because of the natural habitat of Xiphophorus hellerii, they should be kept in slightly hard alkaline water. A pH between 6.8-7.5, with a temperature between 22-24 Celsius and hardness between 100-150 mg/l, suits the species.
Because most livebearers are active species and inhibit in highly oxygenated waters, consume more oxygen and produce more carbon dioxide, with the result that they need a reasonable space in the aquarium.
Furthermore, male swordtails sometimes get aggressive in small aquariums. An aquarium with 110 liters (recommended minimum size) of water is suitable for a group of 3-4 swordtails. Plants are essential in the aquarium for livebearers as they provide shelter for stressed fish and their offspring. The aquarium must be well covered because of swordtails’ tendency to jump.
Xiphophorus hellerii are omnivorous and can eat prepared food and small live-foods (brine shrimp, micro-worms, daphnia, white worms, tubifex). In nature, their diet consists of small invertebrates, algae, insect’s larvae, etc. Swordtails are more carnivorous from other poeciliids such as platies and mollies.
Swordtails are very easy to breed, you only need both genders. Like the rest of the poeciliid family, they give birth to live fry. The male chases the female, and when the female swordtail is ready, she stays immobile for a few seconds until the male fertilizes her. The action is repeated continuously by the male swordtail. It’s interesting that females can store sperm inside their body for months and get pregnant, without a male. The gestation period lasts 4-6 weeks and affected by environmental factors, diet, age and size of the female swordtail. The female Xiphophorus hellerii produce up to 200 young in one brood which is affected by the size and age of the female fish. Keep in mind the species are cannibals and eat their young. A planted tank may help the babies to survive. You can also place the offspring in breeding traps as well as in nursery tanks for their safety. The young reach their sexual maturity during the fourth month of their life

Red Swordtail Male Xiphophorus Helleri.
“iStock.com/ Mirco_Rosenau.”


  1. Earl Schneider and Dr. Leon f. Whitney 1957, The Complete Guide
    to tropical fishes
  2. David Alderton 2005, 2008, Encyclopedia of aquarium and pond fish, Published in the United States by DK Publishing, ISBN 978-0-7566-3678-4
  3. Wilfred L. Whitern 1983, Livebearers, ISBN 0876665180
  4. Xiphophorus xiphidium. Retrieved from https://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=647013#null
  5. Xiphophorus xiphidium. Retrieved from https://www.seriouslyfish.com/species/xiphophorus-hellerii/

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