You'd want something *real* tiny like a neon goby or something like that. Keep on top of your waterchanges as feeding that small of a tank can cause a nutrient problems fast if you don't stay on top of it.
one clown or neon goby like liquid suggests should be ok if you're going LR/LS/WC's. and if it's not going to be a coral/invert environment you could go more, especially if you use a hob with a wet/dry.
as liquid says it depends heavily on your maintaining the nutrient levels but for FO/FOWLR you have more breathing room imo. shrimp are another option.