Tallinu,
From my personal observations, I have almost always noticed this lighter line occurring more around the time when first introduced, and almost always when placed in an area of high light. When I've observed this white line appearing, it starts from the outside margin and works its way in. Whenever I've seen this, I've taken it as a cue to get them into a lower-light area, and this has consistently caused the line to stop and fade back. Again, this isn't a proven theory either, but I've been able to repeat the results in my tank.