Either cyanide or stress.
In QT it might help if it has more places to hide.
As far as learning to eat, I'd guess by this time it already knows how. It should at least look at things floating in the water, maybe mouth it and then spit it out. Pickup some live brine shrimp, that might get it excited enough to feed. Not much value food wise but it's a start.
How big is the fish and the QT?
SteveU