The ammonia has to be coming from somewhere. It does not just appear from nothing. Naturally, it should convert to nitrite, then nitrate. I'm lost as to what would cause it to stall and stay as ammonia. I would get the water tested at the LFS
. You may have a bad kit. You've got nothing to lose by getting a second opinion. If it stays high, it will kill everything in the tank.
If you are not treating the Ich, you still have that problem if/when the parameters get in check. Even if the Tang lives, so does the Ich and any fish you add could get it.