Metynnis argenteus, the "plain-Jane" silver dollars. They school together, don't tend to pick on other fish, and get large enough to avoid being picked too much themselves.
When they are picked on, they are fast enough to shoot across the tank and get away easily.
Once they get to a decent size and age they will regularly add a little protein to the tank (having huge spawns and being egg scatterers). Their tank mates let you know when the silver dollars are ready to and have begun spawning (always following them around and bumping the females a little- kind of like when the snickers bar gets stuck in the candy machine, a little nudge may loosen it up and "plop" down comes the stuck bar/eggs).
