A surprising number of fish utilize internal fertilization and give birth to living young. Females of some such species, such as the freshwater guppy, may produce viable offspring for many months or even years after a single mating. More commonly, the female needs to mate repeatedly if fertile eggs are to develop. Many of the most common freshwater aquarium fish are live-bearers, such as platys, mollies, and swordtails. In most of these species, gestation averages three to four weeks at average aquarium temperatures (the length of the gestation period is affected by the temperature at which the female is maintained). Although the actual internal development of the babies varies greatly among the many fish species, most seem to provide nourishment to the young through a placenta-like organ.

In the aquarium, live-bearing fish lessen the aquarist's workload in that there is no need to care for delicate eggs. The hard part of the process is done by the female. Live-bearing fish are often born at a fairly large size, thus making them easier to feed and raise than their egg-laying counterparts.