You haven't used an effective antibacterial yet.
Amoxicillin----not effective on the vast majority of bacterial infections of fish which are Gram negative
Nitrofurazone and related--not absorbed, external infections only
Metronidazole--most bacterial infections are aerobic, also no effect on Cryptobia
However, I'd be hesitant to use an antibacterial if there isn't any evidence of a bacterial infection. They are the most common infectious agent though.
Cryptobia is somewhat common, especially in discus. I had a fish that stopped eating for 2 months resume eating after treatment with dimetridazole.
There are so many possibilities though. I recommend looking for a vet that sees fish.