Fish that lay eggs show almost no visible signs of pregnancy, although those who bear live fish start to show a definite abdominal bulge, as well as a general lethargy and signs of secretiveness in the aquarium. In an aquarium with several fish, it is fairly obvious which fish is pregnant, because the rest of the fish stay in their normally slender shape.
In addition to their abdominal swelling, some of the species of live-bearing fish develop a gravid, or a dark spot near their anal fin. The male fish fertilizes the eggs while they are still inside the female, and the young fish are born live, swimming on their own in a short amount of time. Fish that give birth to live young include mollies, platys, pike, swordtails and guppies, among others.
Mother fish that are about to give live birth become increasingly reticent, hiding among the castles and foliage in the aquarium. They are looking for a safe spot for the birth to take place without the dangers of prey nearby. As the mother fish continue to grow and approach birth, they increase in size but lose energy. They often swim slowly and cut back on their eating. These changes indicate that the mother will give birth soon.