Sorry, I haven't seen this before so don't know what direction to tell you to go. If it is an environmental issue, I would suggest being sure that the ammonia level is zero, and the nitrite level is zero, and the pH is stable morning to evening. I have never seen parasites cause an issue like this, so I would stay away from most parasite treatments as they are designed to kill the parasites, but most also have some effect on the fish and filters that you wouldn't want to subject them to if not needed.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard