This is a harder option, but I notice all my fish get a little agressive when one or two new fish are added. They all are afraid of losing "their" territory to the new guy. It'd be similar to the Chinese government buying a house next to you and putting up a government chinese flag right next to your house and a sign saying this is the peoples republic of china property. People would be afraid even though the neighborhood is established and everyone knows each other. Maybe a bad example, but, in short one easy to gang up on and keep in it's place. However, I've had really good luck adding 5-8 fish at once. I do it in the dark in the evening, and the next day the fish are all confused.
I imagine it going something like:
"Was this fish in here yesterday?"
Shrug
"Maybe a group of out of towners came in?"
"Wait, are you an out of towner?"
Shrug.
Obviously adding multiples at once is expensive and tank may not support that. However, I have fairly agressive fish that like to be bullies and when I added a few at once, I saw no agression at all. Not even a little. It was like they were so confused that they said screw it the town's gone to crap and went about their day.