While many fish species are oviparous, not all are; some fish species are ovoviviparous, meaning that eggs are gestated and hatched inside of the female fish's body and fish fry (babies) are born live. These live-birthing fish are commonly known as livebearers.
There are several livebearing fish species, and some of these livebearers are popular aquarium fish, including guppies and an appropriately named species called the endler's livebearer. Breeding guppies in captivity is relatively easy, as this species of tropical fish is a notorious breeder. Live birth can clearly be witnessed, particularly with the use of an in-tank birthing box to isolate pregnant females from the rest of the tank.