I don't believe it's a universal rule. There are always jackass fish out there. My tank has retreats for the loaches, and line-of-sight-breakers in the form of driftwood and sloped gravel. I re-arrange it at each water change.
Sometimes you can't avoid insane fish, for sure. But I really believe that the overwhelming majority of cases involve, lets not say taunting the fish with your fingers but, improperly housing them such that they constantly see their reflection and have no boundaries which they might retreat to in the tank. My clowns sleep with with the jag. It's a big pile of clowns next to the jag. hilarious. I've even the loaches swim right up to him while he's trying to grab some krill and 'split it' with him. He never objects.
There's a tiger botia in there [I had more, but they are old, and they are so rare around here that it's just the one now] who doesn't get along with the loaches and get definitely chases that one away, too.
