Oh, Helen that sounds odd

I'm sorry for you and your oscar, seems he's very far gone.
I have no better idea than continuing carefully with water changes to reduce the nitrates, without shocking him. I'd think he had already been so weakened that the medication and perhaps the difference between tank and tap water stressed him only more. But it is very important to improve the water quality, since the poor quality probably was the cause for him getting sick.
Do you use some kind of water conditioner (don't know if this is right but hopefully you understand) when adding fresh water? I forgot to mention it, assuming everybody does. I don't know what you have available, I use Tetra AquaSafe to get rid of chlorines and protect the slime coat. Hope it is allowed to mention it here.
Just don't give up trying, perhaps you can save him. A miracle would be better though

Or anyone with a better idea.
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Gisela
moppel@gismo.owl.de