Hippo has white spots? Nothing else does

[jstdv8]

I think all the regulars here knows the real solution. And it doesn't happen with a cleaner shrimp or a wrasse or just removing one fish to quarantine for treatment.
All he fish must be put in QT for treatment, The DT must be kept fishless while the treatment is happening in the QT. Copper or hyposalinity.
Don't wait on this until all the fish are showing like the tang. These ich will continue to multiply until they take all the fish.

 

[gradymo]

I have to agree with Jstdv8. Once you start seeing white spots on the fish, more than likely the gills are already saturated with them, and just because you don't see it on the other fish, doesn't mean that their gills aren't infected. QT all fish for at least 8 weeks, leave your DT fishless during that time.