You didn't give a tank size, but I imagine that unless you have a large tank, too many clowns could be a problem as well. Clowns have an elaborate hierarchy, basically the female claims a territory (often fairly large), and accepts one other clown to become the male in that territory. The others remain juveniles on the outskirts until a "spot" opens for them to fill, often by the death of the male. With so many clowns in a tight spot, there may not be enough room to have all of the roles, and the clowns could be combining forces to take out the injured one as mentioned above, thus ensuring that they are higher in line.
The possibility of one not being a false could also explain the aggression, although it is not so easy to tell by the inclusion or absence of a spot, or by color so that is a tough call. True perculas often have a thicker black band on the white stripes than false perculas do, but again this is not exact science either.