Swordtail fish are medium-size, colourful fish. Many aquarium enthusiasts enjoy the swordtails because of the sword feature of the male fish, which looks something like a long spike protruding from the bottom of its body along the tail fin. Females don't have the sword feature but tend to be bigger and stronger. Swordtail fish breed easily in captivity and give birth to live young instead of laying eggs. It's important to know whether your female swordtail fish is pregnant so you can prepare the aquarium for the fry.
Write down everything you can remember feeding your fish for the past week or so. These fish enjoy a variety of foods in their diet, but tend to start breeding after they have been fed live food for a period of time. If the list you create shows a large amount of live food, the fish might have bred and your female may be pregnant.
Watch your female swordtail fish, observing any changes in her abdomen's appearance. As the fry develop in a pregnant female, her abdomen will swell. If your fish's abdomen has become noticeably larger, she's likely pregnant.
Look for darkened colouring on your female swordtail fish's abdomen. This colouring will occur just before the beginning of the tail fin, and should look to be roughly the shape of a triangle. It indicates that your female is carrying fry. The dark colouring is actually the eggs and, later, the dark eyes of the fry, showing through the translucent skin of the female swordtail.
Many swordtail owners who only have female fish in the aquarium are confused by females that appear pregnant. However, swordtail females can hold sperm for a long time, using it periodically for several months after their last encounter with a male. If you had a male in the tank in the recent past, or you recently purchased your fish from a community aquarium with males in it, this could be the cause of the pregnancy. Females can use the stored sperm several times. In addition, in a group of several females without a male, a female will occasionally change its sex, becoming a male. This is often the cause of a surprise swordtail fish pregnancy.
Swordtails and other fish in the aquarium will eat the fry soon after they are born, so if you want them to survive, remove the mother to a breeder tank when you notice she's pregnant. After the fish are born, you can return the female swordtail to the main aquarium. The fry can remain in the breeder tank, safe from predators, until they are large enough to fend for themselves.