http://www.chucksaddiction.com/disease.html Most of the disease stuff is lower down.
Rapid breathing could indicate an infestation of the gills, which could be ich, tremadodes, flukes and probably more I'm sure.
I would now be thinking that spot could be indicative of something. If it were my fish, I might try to contact Kelly Jedlicki or Leebca directly. I've been away from RC for awhile, but they were they best people I knew of a few years back.
I can't find it anymore, but Kelly had a chart for dosing antibiotics and prazi orally based on weight of the fish. Depending on the problem, that can be better than just adding it to the tank water.
I've given oral antibiotics to my fish and never had any noticeable effect on bacterial denitrifiers (I see you have some rock in there).
Why not start taking the salinity down? Isn't that less stressful on the fish? But I don't know if you can treat a fish in hypo with prazi or antibiotics.
I know Kelly used to recommend feeding beta-glucan (health food store) to strengthen the fish's defenses. In my experience when you figure out the right treatment, the fish will improve rapidly. Good luck and please keep us posted and if you happen to get that chart, I would love to have it again.