You can do nano fish. It depends on the shape of the tank, but some that work in standard 5 gals are chili rasbora and other micro rasboras, microdevario kubotai, ember tetras, endlers livebearers, least killies, normans lampeye, pygmy sunfish, scarlet badis, sparkling gourami, celestial pearl danios, danio erythromicron (aka emerald rasboras), danio tiniwini, hara jerdoni, dwarf shrimp, and dwarf crays such as CPO's and cajun's.