i'm guessing zebra danios.

a 20 gallon tank is a nice little tank. you can house 20 inches of full grown fish in it... so not many at all.

it's nice to do a little school of fish in a smaller tank... like 15 cardinal tetras (look like neon tetras but the red stripe goes all the way) - a couple of dwarf rainbow fish, perhaps a couple of yoyo loaches for some fun, 3 cories to clean up the bottom of the tank (of course the yoyos will help with that as well) and that would be a full tank for ya. you'd have the red and blue of the cardinal tetras... the pale blue of the dward rainbows... the yoyos are black and white so you'd have some nice colour.

you could of course change that around have add some orange to the tank as well with some rosy barbs (the ones with the long fancy fins are gorgeous) - you could get them instead of the dwarf rainbow fish