For Tangs you usually want to treat them in nothing smaller than a 30 gal
long to give them a little space to swim. You have to treat all
your fish in the tank, not just the new ones. If you don't, they will just get reinfected when put back in the DT.
Usually all the spots will be gone in a few days, after they disappear, continue the treatment for 6 weeks to ensure that no larval stage ich survives. Total time in the QT is approx. 7-8 weeks (6 weeks at hyposalinity, then 1-2 weeks to gradually raise the salinity back to normal.), this will also allow any ich left over in the DT to die as well, giving you an ich free system.