We'd like to see some photo's of the group as well as some individuals.
For all discus to stop eating at the same time usually means water issues or cross contamination. I doubt the food is the cause unless something bad or toxic was included that slipped by.
You performed QT on your new arrivals but did you use a hero from your existing stock as a test? How long ago were they introduced to the main tank?
I would strongly suggest aging your water again, at least until this problem is solved. At least this would eliminate aging as a possible source of your issues. I would also see if you can increase water changes during this time as well. If there is CC then the best thing to do is to reduce bacteria count through water changes.