if you still want to go with a south american type i would put in some hatchets. they will add movement to your tank for sure.what size tank is it?

White cloud mountain minnows love the top. =^_^= They're peacful, and the males have lovely colors when kept well. You need a longer tank rather than a taller one so they can dart about like they like to, and there's a certain male/female ratio you should keep, though I'm not sure what that is.

I second the hatchet's, their lively but prone to jump out tank if not competely covered. Not top dwelleres but danios will liven up any tank and are harmless.

Hatchets are wonderful fish :) If you have sufficient room and filtration, get a small school of them because they don't do well alone. They will feel much more secure with floating plants too. As others have said, they are prone to jumping and every crack and crevice must be covered. Duct tape if you have to!