I agree that the top right corner one looks like its the runt of the pack. To me it seems overcrowding and if you are running the koralia they it might be a bit too much for them.
From what I have learnt over the years discus can tolerate high flow, but prefer to be more peaceful environments.
Your water parameters might be causing some issues, but for me I bought asian discus and over time I managed to keep them in tap water and they were fine. What about feeding? What are you feeding them?
Mine really loved bloodworm, brine shrimp and during their growing periods I would feed them beefheart mix and daily water changes as they are messy eaters and create alot of waste.
Darker backgrounds also set them off but I would say its likely to be overcrowding.