If you have plenty of live rock in your tank, chances are you bio filter is still ok. Hippo tangs are finicky creatures, they do not like their environment changed. When your tank changes in a big way, the hippo gets stressed out and breaks out in ick. Assuming your water quality is still ok, the best thing to do is to leave your tank alone, if you try to catch a hippo in a tank full of rock, by the time you get it out, it will be close to death and you will most likely lose it.
The best course of action was to suck out your sand bed out a little at a time with as little junk stirred up as possible, but since it is already out, just leave the tank the way it is. Feed good food, such as frozen mysis. I found that feeding garlic soaked food really help my hippo to get rid of the ick. If you keep your tank water ok and disturbe the tank as little as possible, your fishes will pull through this with garlic feeding.
This is the method that have worked very well for me, I hope it works for you as well.
Good luck.
Yili