Well, it does seem that sometimes this flashing could be part of aggressive displays toward eachother. I've seen them do it especially with fish that are having rivalries. Can this happen?
The other thing I've noticed on one of my danios is what seems to be bumps near the nostrils and one on top of the head. These bumps appear to be the same color of the fish. BTW, he is a fancy finned zebra danio. Don't know if they specifically are affected by diseas or not. Does anyone know what this could be, and how I could treat it?