Hippo tangs are tricky!! I have had mine for over 5 years and somehow last year he broke out in spots. It went away without treatment. Fast forward to my 300 gallon upgrade. I moved the hippo and a yellow from my 125 to a 55 for hypo qt before they go into the 300 just in case. After a week, the yellow started having issues so I upped the the salinity and shortly after moved them to the 300. After all, they are old and "look" fine. Within 24 hours of being in the 300 the hippo broke out into spots? How is this possible? He has been crystal clear and healthy forever!! I ripped him back out of the 300 and threw him into a 29 for hypo. You may think the 29 is cruel, but my first thoughts were spear and onto the chopping block!!! How can this fish harbor this parasite without showing signs for soooo long. Perhaps there is something else that "looks" like ich. Moral of the story is not only qt, but be careful.
Good luck