Danios in anything under 20g (IMO) just look like bees in a jar. Very high-energy fish adapted to swimming against strong currents in cold mountain streams. Also they need a school of at least 6 otherwise they will try to 'blend in' by schooling with other fish, often chasing, harrassing and nipping them to death.
An awesome danio for a 5g is the Celestial Pearl Danio. They are incredibly tiny (under an inch nose to tail) have a really cool pattern of white spots on a black body w/ red fins and just look awesome in a 'nano' tank display. Any microrasbora will do jsut as well though.