koi swordtails

Green swordtail (Xiphophorus helleri) – a fish with a history

A green swordtail (Xiphophorus helleri) is a relatively large and active fish which belongs to the family of Poeciliidae. In aquariums, you can find a number of breeding varieties that differ not only in coloration, but also in the shape of their fins. Unfortunately, in addition to the well-established breeding varieties kept in aquaria, there is also a whole bunch of accidental hybrids that are created in aquarists’ tanks as a result of uncontrolled reproduction and intraspecies crosses between varieties, as well as interspecific ones with common platy (X. maculatus) and variatus platy (X. variatus). This makes finding quality fish in terms of genetics and phenotype (appearance) no easy task. Of course, the starting form for obtaining these colorful farmed fish were wild fish. It turns out that today wild green swordtail is a real rarity, in huge demand among lovers of wild ovoviparous fish.

Where do swordtails live?

A green swordtail (Xiphophorus helleri) inhabits still and slow-moving waters in Mexico, Guatemala, Hondurasand Belize. The wild population of swordtails has the largest distribution range among representatives of the genus Xiphophorus. Swordtails inhabit a variety of environments. They live in rivers, canals and streams with weak water currents, as well as in lakes and coastal lagoons. Adult fish can be found in open water at a depth of about 30 cm. Young fish and fry choose the coastal zone populated with plants and tree roots. Such places are perfect shelters from predators.

As a result of introduction, the fish can be also found in other parts of the world, where it is considered an invasive species. This is the case, for example, in some US states, Israel, Hong Kong and Australia. It can also be found in Poland, where fish abandoned by aquarists end up in water bodies in city parks. A permanent population of swordtails lives in Bulgaria.

Characteristics of swordtails

A green swordtail (Xiphophorus helleri) is big fish. They grow up to about 10 cm. The fish display sexual dimorphism. Females, compared to males, are larger and bulkier, and have a so-called gravid spot. The distinctive feature of the sexually mature male is the sword, which is formed by elongating the lower rays of the caudal fin. It takes quite a long time to develop a sword. In addition, some males mature later. Therefore, in the case of young fish, distinguishing the sexes is sometimes difficult. Immature males are sometimes mistaken for females. Males also have a copulatory organ called a gonopodium, which is formed from the transformation of the anal fin.

Red Swordtail Male Xiphophorus Helleri

Swordtails are peaceful fish, although males can be aggressive towards each other. So in smaller tanks it is better to keep one male and several females.

Breeding forms of swordtails

Over the years of breeding, a great number of color varieties have been obtained. However, with swordtails, it’s not just the color that is important, but also the shape of their fins. Based on these, the following standards are distinguished:

  • normal-fin,
  • hi-fin – Simpson,
  • hi-fin – delta,
  • long-fin – lyre,
  • long-fin – lyre (caudal fin) and delta (dorsal fin) forms.
normal-fin standard of swordtail
hi-fin – Simpson standard of swordtail
hi-fin – Simpson
hi-fin – delta standard of swordtail
hi-fin – delta
long-fin – lyre standard of swordtail
long-fin – lyre
long-fin – lyre (caudal fin) and delta (dorsal fin) forms standard of swordtail
long-fin – lyre (caudal fin) and delta (dorsal fin) forms

Fish with different fin standards can have different coloration. The strains include: red, Berlin (red with black dots and spots), wagtail (red body and black fins), tuxedo (lower part of the body black, the upper part can be red, yellow, green), koi (albino red and white), pineapple, tricolor, etc.

Wild swordtails

I mentioned the wild form of Xiphophorus hellerii, called the green swordtail. There is one more interesting line of the fish X. helleri “ available commercially – Yucatan”, which came to Berlin in 1975, brought by Günter Daul from an expedition to Mexico. Interestingly, it has managed to preserve the purity of this unique color form for so many years. However, it is worth remembering that other species of swordtails belong to this genus, such as X. alvarezi or the so-called northern swordtails. These are species that inhabit the cool, fast-flowing waters of the Panuco River basin. This group is made up of three clades: Montezumae(X. montezumae X. continens, X. nezahualcoyotl), Cortezi(X. cortezi, X. birchmanni, X. malinche) and Pygmaeus(X. pygmaeus, X. nigrensis, X. mulitlineatus).

These fish are rarely available commercially, as they do not have bright eye-catching colors. However, their uncommon beauty is appreciated by lovers of wild viviparous fish. To get them, it is best to contact the breeder directly.

Sex reversal in swordtails

The ability to change sex has been observed in swordtails. This was first described in 1926 by Essenberg and Harms. However, these reports have been disputed by many scientists. They were finally confirmed only in 1980.

Aquarium for swordtail

The aquarium should be quite large. To provide suitable conditions for a flock of these fish, it is best to choose a tank with a capacity of 240 l. Given the male’s pushy nature towards each other, it is best to keep a group of females and one male in the aquarium. In larger aquariums, several males can be kept, in which case the aggression is not directed at one specific individual. The worst choice is to put two males in one tank. In this scenario, the weaker fish generally gets tortured and dies.

In relation to physical and chemical parameters, these fish are quite flexible. Keep the temperature between 23-25°C. Fish will do well in water with a pH of 7.0-7.5 and hardness of 6-25°dGH.

Plant lots of plants in the tank, while leaving room for swimming. Plants will give the fish a sense of security, as well as the shelter for the fry.

What to feed swordtail?

Their diet is based on small invertebrates. In the aquarium, you can feed them multi-ingredient ready-made foods, such as: America size S, Supervit, Red Mico Colour Sticks, and those that contain insects and crustaceans, such as: Insect Menu Granules Size S, Tubivit, Krill Flake. Their diet should be supplemented with plant foods such as: Super Spirulina Forte, Spirulina Flakes, 3-Algae Flakes. Fish will also readily accept live and frozen foods, such as: tubifex, black mosquito larvae, chironomid and glass worms.

Reproduction of swordtails

Swordtails are ovoviviparous fish. Fertilization takes place in the female’s reproductive tract. Eggs develop in the ovary follicles. Pregnancy lasts about 30-40 days. Embryos take nutrients from the yolk sacs. Just before the birth, the egg shell breaks and the young fish are released outside. A well-fed large female can give birth to as many as dozens of young. Swordtails eat their offspring, so just before giving birth, it’s a good idea to put the female in a nursery. It is a kind of basket from which young fish can swim away from their mother. After mating once, a female can have several litters. Juveniles should be fed powdered foods, such as Micro-vit Basic, Pro Defense Micro Size, and artemia nauplius.


Rafał Maciaszek i inni, (20190, Alien freshwater fish, Xiphophorus interspecies hybrid (Poeciliidae) found in artificial lake in Warsaw, Central Poland, „World Scientific News”, 132, s. 291–299 (ang.).

Elso Lodi (1980), Sex inversion in domesticated strains of the swordtail, Xiphophorus helleri Heckel (Pisces, Osteichthyes), Italian Journal of Zoology, 47:1-2, 1-8,


Ph.D.Eng Aleksandra Kwaśniak-Płacheta

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