Mollies need lots of space, I do not keep mine in anything less than a 29 gallon and actually, I keep mine in a 55. These are fish that love to swim and need their room.
For healthy Mollies, they need lots of room, hard water (no salt), and plenty of algae growing in the tank for them to eat.
Best things to keep in your tank either a Betta, or a school of Dwarf Livebearers, info on my signiture and picture is my avatar. There are few fish I will recommend for a 5-6 gallon tank.