I watched a piece on the TV one day, it showed a little patch of algae that was being tended by two powder blue tangs. They guarded it and ate from it, never straying too far away.
Then suddenly, this large school of convict tangs, maybe 50 members, descended on the algae patch. The powder blues swam around trying to defend their little algae patch, but to no avail, they were hopelessly outnumbered. The convicts ate every last bit of the algae, then moved on. The powder blues were left with no algae. If a fish can look sad, those two were it.
A very interesting piece, in my opinion. It shows why powder blues are somewhat aggressive to other fish, and it shows how a group of convicts work together.