the submissive ones get stress stripes when another girl swims by, then as the other girl swims away, the stripes fade. i think in this case, its more of a sign of submission than a sign of actual stress and fear.
i know its not a water quality issue, as the alpha (the larger, and darker devil girl) is jet black with blood red fins all the time, constantly flaring and strutting her stuff. her fins have never been perfect from introduction to the tank, though, as occasionally the other not-all-that-submissive girls get fed up with her and spar. since they all occasionally squabble with her, and not with each other, she is the most torn up. all of it is superficial though. no damage to anything but the ray extensions which are the easiest to get at.
Eheim Pimp #296
Canon Pimp #67