Now that the thread's been resurrected, I'll chime in too. I think the "understand that he's a teenager and possibly mad at the world" approach would have worked. A little education could have been helpful. He probably didn't understand that there can be a large behavioral difference between different species of fish with a common family name (i.e. cichlids, rainbows, etc.). As a matter of fact, many of these fish aren't even in the same order, let alone genus. I just finished re-homing three of my black banded rainbows (Melanotaenia nigrans) after they went feral and killed a dwarf gourami, three other Rainbows, two neons, 2 peppered coreys, a blue ram, and a Kribensis. Given that, I would have classified that particular species of rainbow as at least Semi-aggressive, if not flat out aggressive. On the other hand I can't imagine that happening with something like a school of threadfins or celebes.