You said it was a new tank, but then you said you had the tank for one year and never lost a fish to ick. Which is it? Assuming you started your tank over and went through the whole cycling process, then it's a newly cycled tank and you shouldn't add any delicate fish like a tang to the tank. Start with something hardy first.
To be on the safe side, wait at least 4 weeks for the ick to totally die off.