Instructions
Examine the tail of the fish. The tail of the female is uniformly rounded and greatly resembles the tail of a platy. The adult male's tail, however, gives this species its common name "swordtail." The lower part of the adult male's tail is exceptionally long and pointed like a sword. In babies, you can see this sword start to develop by the time the fry are halfway grown at 1 to 2 inches in length. Adult swordtail males are about 4 inches in length not including the swordtail, reports "500 Freshwater Aquarium Fish."
Look for the gonopodium on the belly of the fish. This is a modified fin found just behind the lower fins and found only on the males. It's visible on the young and before the swordtail begins to develop.
Look for swordtail fish that fight with other swordtails. These are usually the males. Most tanks can only handle one male. Groups of males will not live peacefully, instead they will chase each other and fight.
How to Tell a Male Baby Swordtail From a Female
The swordtail fish (Xiphophorus helleri), much like its closely related cousin the platy, are generally a peaceful and hardy fish, reports Joseph S. Levine in "The Complete Fishkeeper." Easy to breed, they also make a good fish for the beginning fish breeder provided you have a male and female fish in your tank. Behavior and obvious physical differences between the male and female swordtail fish, even as babies, will help you differentiate the sexes.