Ok this does happen in established tanks. Especially with smaller stunted fish. So many thing discus just have in them already. Most of the time the immunity system keeps these things at bay. But stress does bring them out. Sometimes meds work sometimes moving the fish works and sometimes nothing works. I’ve found this is the nature of these fish.
Danios are cold water fish and just shouldn’t be with discus. Discus should be at 82-86.the higher side to get them eating. What other fish do you have in the tank? How many discus now? How often and how much water are you changing.