Guppies and mollies are a natural pairing — two livebearing species from the same broad family of Central American waters, asking for nearly identical conditions and bringing the same busy, surface-oriented energy to a tank. Where many mixed communities involve one species tolerating the other's preferences, this one is a genuine overlap: set the tank up correctly for one of them and you have, with small adjustments, set it up for both.
Shared water is the foundation
Both fish evolved in hard, alkaline, mineral-rich water and look washed out and disease-prone in soft, acidic tanks. Warmth in the upper seventies Fahrenheit suits them, and both appreciate a diet with a vegetable component — mollies in particular graze algae and soft plant matter through the day, which is why a molly in a mature tank is constantly picking at leaves and glass. Mollies are the larger fish, reaching three or four inches against the guppy's two, so a twenty-gallon tank is the practical minimum for a mixed group, with thirty giving the mollies proper swimming room and diluting the waste from two prolific species.
Managing the males and the fry
The real work in this pairing is social, not chemical. Males of both species court relentlessly, and a lone female will be harassed to exhaustion, so keep at least two or three females per male in each species. Males will also display at the other species' females, which is harmless theatre as long as female numbers stay high. Both species produce fry every month or so, and a mixed tank fills up faster than beginners expect. Floating plants and dense corners let some fry survive; a fine-mesh breeder box or a small grow-out tank lets you raise a chosen batch deliberately. Decide early what you will do with youngsters — local shops and other keepers often take them — because an unmanaged livebearer tank becomes overstocked quietly and then all at once.
Feeding a two-species surface crowd
Both fish feed at the surface and mid-water, so one quality flake or micro-pellet ration covers the tank, with the vegetable side handled through spirulina-based foods, blanched greens, and the algae the mollies harvest themselves. Feed small amounts twice a day and make sure food reaches the lower swimmers before the surface crowd clears it. Watch the mollies' shape as your water-quality gauge: a molly that shimmies in place, rocking side to side, is a molly telling you the water has drifted out of the hard, warm zone both species need, and a test and water change are due.
Youngsters moving to new homes
A healthy guppy-and-molly tank produces more young fish than one household can keep, and rehoming them is a normal part of keeping livebearers well. Recording a batch's parents, birth timing, and what the fry were raised on in KinsLe's Transferrable Profiles means the keeper receiving your young mollies or guppies starts with their real history in hand, and your own tank's records stay accurate about who stayed and who moved on.