Sick Blue Reef Chromis

I just started volunteering at a state park as an animal care helper. This fish came from a roughly 500+ gallon FO tank. In the tank there's 7 other blue reef chromis, a French angelfish, some kind of smaller angelfish (I forget what kind), an Atlantic blue tang, and a couple of other semi-aggressive fish (again, I'm new to volunteering there so I can't recall all the fish in the tank ATM). I haven't seen any aggression between the fish, and I've been told that all the fish in there are fairly peaceful, but some aggression is obviously possible. Nitrate is at around 5ppm and I assume that ammonia and nitrite are at zero; the tank is well established and at least a year or two old.



8 days ago this chromis was seen just floating around belly-up, so it was placed in the QT at the facility. It hasn't eaten at all since then and has mostly just rested on his side, barely breathing, in the QT. Also, the fish has developed a white cloud that covers one of its eyes. This may be the result of some infection, but it may also be the result of the fish rubbing its eye on the glass as it drifts around in a really sad way along the bottom. 2 medications have been used (I don't know which exactly, but I was told that one of them was supposed to help cure a fungal infection). None of us have been able to ID the affliction, but that's where TRT comes in. What do you think is ailing this fish? Fungal infection? Bacterial infection? Some other parasite? Swim bladder issues? Any thoughts would be appreciated.