Hi welcome to AA. It could very well be a parasite, the symptoms are suggesting that. I would first do a pwc
of 50% then add some meds such as Jungle's parasite clear. Since the fish isn't eating medicated foods will not help.
Do you have live plants in the tank? What kind of test strips are you using? The reason I ask is the lack of nitrates. Established tanks without being heavily planted should show some nitrates.