Well I found my lfs has hex-shield so will feed them that and hope with the water changes and all will do….
You can throw all the meds you can find at that oscar, and they may act as a temporary bandaid, but unless you get a hold of the underlying cause of the HITH, (the most important part of your last statement), it won't matter ( the nitrates)
HITH is a chronic condition, it didn't happen overnight although it seemed to just appear, no....its been festering for months.
If you think of it like......40 ppm nitrate is the equivalent of your tank being filled with 40% urine.
How long would you swim around in 40% piss before you got a skin condition (sorry if that seems a bit harsh)
I wish every potential oscar owner would read this thread before they get one.
I have seen this so many times, with scarred up oscars turned in to LFSs.
If you have a dog, it needs to be walked everyday, oscars (and any large cichlids) version of a walk, are large frequent water changes.