Some times hith can really kill their appetite and ideally you want the fish to actually be consuming the medication through food or something. So if medicating the water doesn't work (which it sometimes doesn't) you're gonna have to soak the food or something. And if he's not hungry theeeeen... It's all related.
If I were you I would dose the tank 2 or 3 consecutive days then do at least a 50% change. Then repeat.
For long? I don't know. I know some guys that bring in wild caught fish treat them for weeks when they have intestinal parasites. But I'd do some research on it. And, personally, in dealing with a green terror that has parasites, it's work haha.
Oh and I think metro is used to treat hith. Two birds...