Livebearers
Live-bearing aquarium fish are fish of certain species which are ovo-viviparous. This means that while the fry still incubate inside eggs, these eggs are not expelled at an early stage of their development, but are retained within the female's body and the fry are allowed to hatch inside her before they are expelled.
Some of these fish, for example the guppy, Platy, Swordbearer and Mollies are among the most popular aquarium fish. They have gained their popularity by being colorfull and very easy to care for and breed which makes them ideal fish for people how want to start keeping aquarium fish.
Placental mammals, on the other hand, are viviparous, and have dropped the egg stage altogether.
Almost all popular live-bearing aquarium fish belong to the family Poecilidae, that is, the guppies, mollies, platies and swordtails, but half beaks and some species of sharks and rays also bear live young.