The symptoms your Oscar is exhibiting doesn't sound that serious to me. If you could take a photograph, that would be very helpful. Have you actually tested your water? If you could place the test results, we can be absolutely sure everything is okay.
Myxazin & Maracyn 2 are two proven medications that will treat a variety of ailments including fin rot. However, I don't like to add medication unless it's absolutely necessary.
The first Oscars I had a few years ago developed a film over one eye. I think this was due to stress, or maybe changes in water conditions. The film peeled off after a few days, does this sound like what your Oscar has got?
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