http://elmlaneaquatics.com/fish/tropical/livebearers/

Swordtails (Xiphophorus hellerii)

Swordtails are very popular live-bearers that come in a variety of colours.  They were originally found in found in North and Central America, but are now commercially bred much closer to home.  The female looks similar to a platy, while the male has a splendid ‘sword’ extending outward from the bottom of its tail fin, which grows gradually as it reaches maturity.  The ‘sword’ doesn’t appear to have any purpose, beyond being an eye-catching accessory, but it looks cool, and young males give the impression of being quite proud of them!

Swordtails are easy to breed and fast-growing. They are hardy, peaceful and lively fish; they will live in small, loosely connected tribes of their own species, with other live-bearers and other small community fish such as danios and peaceful tetras.   Males can be aggressive or territorial with other males, but this can be avoided by having a large enough shoal, or, alternatively, just keeping one male with a group of females.  You need to keep more females than males in a mixed  tank anyway, as even just one male will pester a lone female constantly.  They like a well-planted tank (including floating plants to protect newborn fry from being eaten by its parents) with lots of room to swim around: they do not stick to one particular level of the aquarium, but explore all over the space available to them.

Swordtails will eat all kinds of fresh, frozen, and flake foods. They also enjoy occasional brine shrimp as a treat, and will enthusiastically nibble vegetable treats, such as courgette, added to the tank for a few hours.

Size (fully grown): up to 16cm (6″) in the wild; more likely 10cm (4″) in captivity.

Water temperature: 18 – 28C

Water pH: 7 – 8.5