How were the fish acclimated and how were they added to the tank? How long have the sponges been cycling prior to adding the new fish?
I'm worried with all the medical suggestions here. Especially treating anti-bacteria in a newly established tank, with new fish added, with likely a "mini-cycle" as Iminit mentioned.
My thoughts:
-Only true symptom we see is the white dusting as described on the fish and images show it on fins as well. This is typically ich, which is better treated with ich medication (I use ich-x and have good success) and higher temps (ich life cycle survives at a lower temperature so keeping temps above 85F will stop the life cycle). This could have came from used media or other supplies (nets, hoses, trash cans, etc.). Where it came from really doesn't matter, because it's in the tank now.
-Clamped fin and damaged fins can happen during the movement from shipping bag to the tank when using a net. These will heal with clean water changes. If it is progressively getting worse and not better with water changes, then likely could be a bacteria issue.
-Going a little darker (or peppering) and/or not eating in newly added fish may not mean cross contamination right away. If you have a fish going dark, dark and moving individually to the corner, then cross contamination may be the issue. To me, It is more likely just stress with a new environment, new water and water change regimen, and currently being medicated. Wattley has all his fish on a central system with autowater changes. I wouldn't think they would be cross contaminated with each other if all coming from the same source, but I could be wrong.
In my opinion, I'd give it more time and fresh water changes as you have been doing. I wouldn't over salt or anything yet (for new fish, I'll use a tablespoon per 10 gallons). I'd likely raise temps to 85F and continue treating for ich for at least 7 days, maybe 8 days. After, I'd drop the temperatures back to 82F gradually to see if the ich is gone for good.
Sorry you are dealing with this as soon as you get back into keeping discus!
Phil
Edit to add: So I don't know why I didn't see the second page of posts, but if that is white stringy poop, that would be another true symptom and probably another reason why they are not eating. Probably why Wattley has a video on treating with metronidazole... If you talk to your rep at Wattley and/or Gabe, show them the images of the poop and request some metronidazole. It's not like it came from you and your feedings when they aren't eating. They should honor that and give it to you on the house in my opinion. Watch the video on youtube from Wattley Discus channel and use those talking points.
Good luck!