They will breed in most conditions, simply provide a decent sized, well maintained tank with hard/alkaline water and some vegetation, either real or plastic. I have bred them in a ten gallon tank, and raised the babies for quite some time, and that was so long ago I don't quite recall what became of all of them. In my experience, the larger Xiphophorus variatus is easier to rear and produces more young, but both species can be bred/raised with little difficulty. Feed the fry crushed/powdered flake or similarly tiny foods, and keep them in a seperate rearing tank for best results. In my experience it helps to have other small fish to eat the left over food, such as Corydoras or small guppies.