For a 1/2 to 1 gallon tank, I'd say a betta or paradise fish.
For 5-10 gallons, small tetras, *neons, cardinals, glowlights* corycats, white clouds, and danios *not giant* are all perfect and may even breed.
For a 20-30, barbs, silver dollars, larger tetras, and some smaller cichlids are good.
55 is good for an Oscar or a tropical community, and from there up the sky's the limit. 8)