Yes, platies and mollies can live together. Both are peaceful livebearers that want similar water: temperatures of about 72–78°F, a pH of 7.0 to 8.2, and moderately hard to hard water (roughly 10–30 dGH). A tank of 20 gallons or more holds a small group of each, and they cannot crossbreed because they are different genera.
Platies stay around 2 inches and mollies reach about 3 inches or more, with sailfin mollies growing larger. That size difference rarely causes problems. The larger issue is stocking, because both species breed fast and eat a lot.
How many platies and mollies can you keep together?
The old rule of 2 to 3 gallons per fish only works for a few small fish. Plan on about 20 gallons for 2 to 3 platies and 2 to 3 mollies, and more for sailfin mollies. Seriously Fish lists minimum tanks of roughly 14 gallons for platies and 21 gallons for short-finned mollies on their own, so a mixed group needs more room. Keep two or three females per male of each species, because males chase females constantly and may wear them out.
What to watch for
- Soft or acidic water. Both species weaken in soft water. Look for clamped fins, shimmying, and white fungus patches. Add mineral hardness (not just salt) if your tap water is soft.
- Overbreeding. Both species give birth to live fry after a 4 to 8 week pregnancy, in broods that can reach 80 (platies) or 120 (mollies). Plan for what to do with the extra fish, or keep mostly females.
- Male harassment. If one fish hides all day or has torn fins, add more females, plants, and hiding places.
- Weak stock. Mass-bred platies and mollies are often less hardy. Quarantine new fish for two to four weeks.
In addition to mollies, here are some good tank mates for platies:
- Guppies
- Tetras (pick hardy species that tolerate harder water)
- Swordtails
- Rasboras
- Cory catfish
- Zebra danios
- Bettas (only calm individuals; livebearer males can pester them)
- Minnows
- Plecos (small species)
- Gouramis
- Angelfish (they may eat fry and small adults)
- Otocinclus catfish