Welcome to the forum!! Sorry you're having issues but this appears to be a simple case of hex, brought on by irregularities in water, ph and temps. Hexamita can take advantage of smaller discus and even larger ones that are experiencing different forms of stress.
I would raise the temp to 93 and add some regular salt to the tank at 3-5 TBS per 10g. I would do regular daily water changes and I would start aging your water. Water straight from tap contains high levels of Co2 and can cause ph swings that create the exact stress you are talking about.
I would also order some metro and treat them for 10-14 days. It's one of the best meds to fight hex and is something I always have on hand. Meds like General Cure and others that contain metro are weaker versions and not nearly as effective. You can order pure metro here https://www.jehmco.com/html/medications.html