Welcome to the forum! Sorry it's under these circumstances... A couple of observations and questions...
Are they from a reputable seller or from a LFS?
When you added now fish to this tank, did you quarantine them?
What were the symptoms from the ones you lost? Similar or different?
You wrote that there is 1" of gravel on the tank but the photo shows bare bottom. When/why did you make this change?
They have been medicated pretty heavily. I would not use any more medications at this point and I would not listen to most LFS's for medicating advice. Everyone's first instinct is to worm discus and Levamisole is pretty harsh on them, especially if other meds have been used around the same time.
You're using HOB filters? Are you using any charcoal media? For the short term, I would remove all media and use the floss only.
You're aging your water like a pro! However, I would add the prime to the tank at the time of the water change rather than adding to the barrel. Also, I would increase water changes to at least every other day. The way you age your water, you can probably go up to full 100% daily, but 50% should be fine.