Over-filtered? I suppose so... the filter is meant for a much larger aquarium. It's a Hang on Back or HOB.
Danios are natural schoolers? I don't know but they break up and swim around then come back especially when it's time to eat. Maybe more Barbs would do the trick, they seem really scared and stressed.
As I said, they don't do anything, just swim around. The female is worse, she doesn't move much.