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I agree with what GodFan said. It depends on the betta, and the molly/platy. Some mollies can be very bossy,and some bettas are not good community residents. In general, assuming you have a community-friendly betta, I think either mollies or platies would be fine as tankmates. ocminpin currently has a betta with her mollies and it works out great. I know other people with a betta and platies, so I know that can work out too.