Vampire double swordtails are a rare and expensive ornamental fish. They are a selectively bred strain of the double swordtail, which is native to Mexico, Guatemala, Belize, and Honduras. 
Vampire swordtails can grow to be around 4 inches long, with females being larger than males. People breed them by crossing two parents with the expressed gene to produce 1/2 that is the same as the parents, 1/4 that is normal, and 1/4 that is "super" and cannot breed. 
Swordtails are omnivorous fish that eat both plants and meat.  They are social fish that thrive in groups and should not be kept alone for long periods of time.  Swordtails can be kept in a community tank with other tropical fish. The minimum tank size for swordtails is 10 gallons, and the water should be between 64°F and 82°F. The water should also have a pH range of 7 to 8 and a dGH range of 12 to 30