Look in the nano forum to see what those guys and gals keep in their small tanks. If it was me I would probably put a couple of shrimp instead of fish.
If you have to have a fish, a neon goby would be best. Very small and not a big bio-load.
If you want something nicer looking but still small, a clown goby would be neat.
If you're willing to relocate in a few months and you are not scared of watching water quality, you could try a pair of small clowns or a firefish. But both would need a bigger tank in a few months.
Since you siad it's a frag tank, you won't mind relocating to your main tank later right?
