Swordtail

Swordtail

The Swordtail, Xiphophorus helleri, is a very popular fish from the rivers and streams of Central America on the Atlantic side. The Swordtail is related to the Guppy and is as easy to breed.
It's larger than the Guppy reaching four and a half inches (12cm) long.

Water Conditions
The Swordtail is a tropical fish which can take slightly colder conditions than many tropical fish.  A temperature of 24̊ C (75̊ F) is suitable for a mixed tank including this fish. Swordtails like harder water than most tetras, and has a high tolerance for salt, so it isn't a particularly good tank mate for tetras. Salt can be used for preventing and curing some diseases, but beware if trying this. Swordtails bred in high salt (sometimes half the salt percentage of the sea)  usually have little resistance to diseases and can die when put into normal fish aquariums.
The Swordtail generally prefers slightly alkaline water. For a mixed tank including Swordtails I suggest neutral (7).

Food
Swordtails are omnivores and eat both live food like mosquito larvae, daphnia and baby fish as well as some algae. In an aquarium they will eat any normal fish food.

Companions
The Swordtail is not a strongly schooling fish, so large groups are not necessary, but they can be kept and look very nice. Two males may cause problems to each other. They don't have stand up fights like Siamese Fighting Fish, but unless distracted by enough females may squabble a bit.
Swordtails are a community fish and suitable companions include the other smaller peaceful fish such as, Guppies, Endlers Guppies, Neon Tetras, Peppered Catfish, Siamese Fighting Fish, White Cloud Mountain Minnows and Zebra Danios. Not all these common companions are compatible with each other. Most of these fish will eat baby Swordtails.

Breeding
Swordtails are livebearers like the Guppy and the Great White Shark. Like both these fish the Swordtail will sometimes eat their own babies. If the Swordtails are kept in a well planted tank without crowding there is more chance of the babies surviving. The gestation period of Swordtails is about 28 days.
For breeding I suggest a ratio of three females to each male. Only the males have the extension of the tail that gives the fish its name. Experiments show that female Swordtails prefer larger males with a longer sword. Many aquarists do as well. Immature males look like females. An early developing male will usually end up as a smaller fish than a late developing male. Before the sword develops, the sex can be seen by the developing gonopodium. This is a modification common to others in this group of fish. The anal fin under the middle of the fish becomes elongated and is used to transfer the packets of sperm, called spermatophores to fertilize the female. The gonopodium is functionally similar to the penis of mammals, but structurally quite different.

Sex Change
When they get older, some female Swordtails will change into males. Not all these males are fertile, but any babies the 'male' fathers will be female. This ability is hereditary.

Pest Fish
Never release your pet fish or put them in the position of being accidentally released. The Swordtail has got into waterways in several places, and is causing serious problems to the native fish. In parts of the Brisbane River, introduced species make up over 80% of the total fish. These are not all Swordtails, but in sections they are the most common fish. In the Brisbane River Swordtails six inches long have been caught. For an extreme example of the damage introducing other species can do, see the Lake Eacham Rainbowfish Fact Sheet for an account of the Extinction of the lovely Lake Eacham Rainbowfish.

Sources

I gratefully acknowledge the following sources of general information about the swordtail. Mary Reed of Ohio University, Sexual selection in the Swordtail and the evolution of ornaments, Department of Fisheries of the Western Australian Government, Aquatic Community, Animal World,  liveAquaria.com, and turrbal environment
 
Steve Challis