Before you stress all of your livestock out by trying to take them all out, how long has your hippo tang been in your tank.
I have kept tangs for several years now. I have never had a QT tank. Tangs almost always get ich. I have 6 tangs in my tank right now, including two hippos. Three of them got ich. I kept feeding them twice a day, gave algae sheets, and now they are all healthy and swimming fine.
I know this goes against many people here, but I don't beleive in QT's unless there is an extreme case of something. I have talked to many other hobbyist that have been in this for 20+ years, some will say it does more harm than good taking a fish out of the tank (stress), and some say it has helped them.