Sorry for the short response earlier, I was pressed for time.
For many years a lot of people blamed Hexamita for HITH. Hex is a parasite, and a very opportunistic one. it will quickly attach to open sores on fish, and HITH is the perfect environment. Metronidazol kills Hexamita, and many other parisites very effectively, and is IMO a very good med when used within it's capabilities. However some cases of hith show no evidence of Hex, and most do not show hex in numbers capable of doing the damage found on the fish. When Hex is killed, Hith remains. It is generally treatable, and doesn't have to be a huge problem, but it will remain no matter what is done, and as mentioned above it will show up and recede based on several factors.
The two primary theories are water cleanliness as mentioned, and nutrition. These two factor go hand in hand very well and that makes it hard to seperate them. This article will give some insight on Hith and ways to treat and controll it. I am not totally sold on vitamin supplements, but do use them somewhat with my big cichlids. My personal feeling, is that Goldfish are the biggest contributor. I've never fed Goldfish to my oscars or anything else for that matter. I use feeders as treats, and never use any fish but guppies or the occasional rosy red. I keep my live offerings very limited, mix multiple foods, and kepp my tanks extremely clean. I have never had a case of HITH develope in any fish I bought young and raised. All of my experience with Hith has been with adopted fish. Most of which Came from nasty situations.
Now most importantly, The lethargy you describe is seldom seen with Hith except in severe cases. I would look at other issues. What are your water parrameters, how much maintenance, what kind and how much filtration. Does your tank match you tap in parrameters. As said a 100% water change can be a bad idea if your tank and your tap are not matched. the easiest way to keep them matched is through frequent water changes and cleaning. but either way testing is needed. We might be able to help more if we know more about the tank. Oscars generally aren't hard to keep aside from the need for good maintenance. and of course Pics will help a bunch.
dave