Adding another Angel

[FishyFishy!]

All of my experiences have varied fish to fish. All of them are different, just like humans. Some will be nasty, and some will be awesome. I would personally just try a different fish.

Acclamation box sounds like a good move. You could also try removing the aggressor for a while. Add the other one to the display for a while so it can establish some territory, and then try the aggressive one in the acclimation box.