Exactly what I was thinking and wanted to hear. My oscar was starting to adapt to the bad water conditions and all of a sudden he got clean(er) water in there. Good thing he is a very hearty fish, hopefully he'll pull through it.
As I have several oscars I know that from time to time they get moody and sometimes go a bit refusing to eat. Try other food options...beef heart, earth worms, crickets. Seeing as how he is in a 55 on his own, is fine...however lacking on his water changes is just unfair, harsh maybe but he relies on you to keep him in clean water I dont know about your oscar, but mine get silly when im in the room to get my attention, splashing, spitting rocks, knocking the lid off. Usually its begging for extra food but I know they watch me because I can walk around the tank and they follow me LMAO. Good luck and I am curious to see photos of this oscar if you can post some