Livebearers!!!
They will breed of u have a male and female
Or is u get just females one might turn into a male to keep the process along
U might often buy a livebearer Prego so be ready for fry!
Livebearers include
Mollies
Guppies
Platys
Swordtails
And endlers
Most of which are a few bucks a piece and are common, small , and quick breeding
See the link I my signature for more info!