Agree, that looks to be HITH, and unless you get to the root of the problem it will continue as a chronic disease, and all the medication in the world is just a temporary bandaid.
To stop it in its tracks, you need to do enough water changes per week (in fact daily) to reduce nitrate to below 5ppm, and clean filters often enough so they don't spew nitrate back into the tank, and they do, when not regularly cleaned.
In a tank as small as a 55 gal, daily 25% would not be overkill.
In fact in such a small tank, 50% or even 75% per day water changes would not be overkill..
The daily waste output of even 1 oscar, without any other fish in a 55, will easily overwhelm that tank and cause chronic HITH.