The only fish suited to a 5 gallon are either [not both] (a) a single male Betta, or (b) a group of one of the nano fish species. Nano fish are very small, usually less than 2 cm full grown, and include fish like the Ember Tetra and the dwarf rasbora species in the genus Boraras. These fish will be wild caught, so before considering any of them we would need to know the GH (general hardness) and pH of your source water. Nano fish have high metabolisms beecause of their size and they have considerably less tolerance for any adverse environmental aspects. "Environmental" here refers to all aspects of their environment, from water parameters to decor to other fish.