Did either fish have any marks on them or anything out of the ordinary physically? Did you happen to qt the tang before you put it in? They are very easily stressed especially in small tanks. Sometimes ich can present itself due to stress.
Your water seems spot on..any chance of a ph or temperature swing?
I'd be concerned for the rest of your stock...
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.