Some fish require buddies. Corys, loaches and otos really like numbers in their schools in order for them to keep healthy and happy. This is where lots of species research prior to purchase comes in handy.
Oh, and toss that 1 gallon per fish thing. It's a bunch of hogwash.
75: blood parrots, featherfin cat, emperor tetras, turquoise rainbowfish, BN plecos, japanese trapdoor; 46: WCMM, gold inca snails, ghost, bamboo & amano shrimp, kuhli loaches, rummynose & ember tetras, endlers, platies, flame gourami, guppies; 16: pygmy, peppered, loxozonus corys, otos, assassins, RCS ... ~ Research PRIOR to purchase.... ~