One clownfish
Or......a goby/pistol shrimp pair....THAT would make a very interesting tank ! Toss some ornamental shrimp in there as well and you'd have a nice little setup.
Try this link out.....it has a list of suitable fish for various sized nano tanks. I imagine there'll be plenty of disagreement here and there, but ....it's a place to start.
lgreen's Ultimate Nano Fish Guide - Nano-Reef.com Forums
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