There are times when angels getting in breeding mode, or becoming agressive over something will get a distinctly fine red line across their dorsal ridges. It will come and go spontaneously with their mood. (angels can be a pain sometimes!)
So, before you treat for any red lines, look for behavioral symptoms that should also occur somewhat concurrently. Are they lethargic? Eating normally? Hiding? Are their fins planked? Coming to the front to beg for food? Has their color faded? By that I mean FADED out, not just a color change that sometimes also occurs in these fish. For instance, my silvers will sometimes lose their stripes completely when stressed (trying to catch them in the net), but when they are feeling feisty they develop dark circles around both eyes and the black becomes MUCH more pronounced.
In general, if there appears to be no other symptomology, I'd do water changes, observe, and see if the line comes and goes before treating a whole tank of angelfish.
Sorry if this is confusing.

Angelfish ... gotta love em!