I sort of buck the fallow tank trend. Here is why: ich is a naturally occurring parasite in marine and freshwater. In natural ecosystems it doesn't pose a huge threat because fish move away from areas where the parasite has settled and will re-emerge. Although in our aquariums ich can essentially, "take over" the situation. Although, in my personal experience nutrition is the best weapon against ich. Garlic is a known parasite preventative. File fish are not highly herbivorous such as tangs. Although, soaking any food, especially frozen in a garlic enriched formula, (I recommend Brightwell Aquatic's Garlic Power) will allow the animal a nutritional opportunity to combat ich.
The bad news is that this approach takes time, other fish can become effected by the parasite. Filefish are sensitive to copper and sometimes hyposalinity. I would take a shot at the nutrition route. Fish tend to build an immunity to ich, lose that immunity over time then rebuild it.