Hospital Tank Question for Ich!

theres always ich in your aquarium, only a weakened fish will succumb to it, the tangs are particularly susceptible because they don't have a thick slimecoat, if i was in your position i would raise the temp and wait a few days, i would use those few days to set my quarantine up permanently, then if there was no improvement i would move the hippo to qt and medicate, if you have or plan to keep corals i really wouldn't medicate the main tank,, since you have a hippo this probably won't be the last time he gets ich assuming he makes it.
more important than any of this is finding out what stressed him in the first place. my shot in the dark guess would be temp fluctuation for this time of year

 

this is a great idea also, you'll see them fall off

 

I would never fresh water dip a fish, especially one that's already stressed out.