Well, even at 3" tangs require a lot of swimming room, or they will get stressed, which doesn't help with treatment. Honestly, I think your current 55 is too small, and may have contributed to the outbreak, but that's another post...
I'm going to say get the biggest
QT you can. You're going to have to keep a very close eye on water parameters here.
Ammonia,
nitrite and
nitrate WILL rise, and you will have to do very regular water changes to keep them in check, like daily. Any
copper based medication should be fine for treatment. You'll need o follow the directions exactly, and probably will need a
copper test kit as well.
In order for the cysts in your display tank to hatch and die, you're going to need to "fallow" your tank. Meaning, the fish will have to be in
QT for at least 6 weeks, 8 is better, with no fish at all in your
DT for the ich to host on/in.
