You most likely arent fully cycled if you only had it running 2 weeks. :-(
I also agree about the tang, a smaller tank can definitely add to the stress of going into a new tank.
If it's ich now is the time to catch it and treat in order to save the fish. The tang should go back to the store and you can treat the remaining fish.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.