He mentioned it was a 200gal 6ft in his opening post.
As for the Oscar, I agree with the others in that your water changes are too small and infrequent. Oscars are messy and 20% a week wont cut it, especially with other tankmates. Your other fish seem "healthy" because they are hardier than the Oscar. IME Oscars are very susceptible to these kind of infections.
You mentioned hes the newest addition and you might have gotten a "messed up Oscar". How Long ago did you buy him? Could it be possible he was sick when you got him? How did you introduce him to your setup?
If you've had him for more than several days and he just started displaying sympstoms we'll have to assume its on you.
Treat with salt and feed with salt like they advised before. And when everything is back to normal do bigger, more frequent WCs.