Yeah what I'm saying is even a small hippo tang will have the tendency to get stressed in a smaller tank. Hippo tangs even moreso than other tangs.
Did you remove all the fish in the tank and treat them all? Or just the tang? You know if you only treat one fish, it will get ich again as soon as you put it back in the tank with the other fish? Once you have ich in your tank, every fish has been exposed. And you have to treat every single one of them to get rid of it. If you are only treating the tang, it's pointless. It will get sick again as soon as you move it back in the tank.
And the angelfish probably got ich because your tank is too small for that fish too :( Ich is opportunistic. Most fish carry it all the time unless they have been specifically treated for it. They will only show signs of ich when stressed or unhealthy. Like when they are moved to a new tank, are put in a tank with poor water quality, are fighting with other fish, or are put in a tank that is too small to meet their needs. I suspect your ich problems probably stem from all of these reasons.