I should have said this before, but welcome aboard, Man.
Tangs are truly addictive and somewhat sensitive fish. I started off in this hobby the wrong way, and ended up with a full tank crash from ich shortly after starting my tank. Trust me, I've had a handful of butt chewings come my way as well, but used them as learning lessons, and both my fish, and I have benefited from it greatly.
In my original post, I asked about QT. With what I've learned, if I were in the position you are in right now, I'd buy a separate tank to set up temporarily, and move all of your existing fish to it. Not saying that your current tank has ich, but if your fish weren't QT'd and/or treated, it may (thinking about future fish purchases...). With all of your current fish in the new temporary tank, this temp tank can serve as your QT. You can monitor and treat your current fish if needed, as well as any new fish you purchase.
While your fish are in QT, you can add all of the coral and inverts you want to your existing DT.
You will have a tri-fold benefit from doing this.
1. You will have all of your fish in QT, and a way to QT your new fish.
2. Your current DT can sit fallow for a minimum of 9 weeks to kill off any ich that may exist in your tank.
3. Your current DT has a chance to age a bit...
The aforementioned can be done concurrently, and you won't have to be without fish in the meantime. Your taste in fish strongly urge the need for QT, and you want to make sure that your DT is healthy for any fish that you choose to add to it...
You can find a 75g tank on craigslist for next to nothing, and it doesn't have to be drilled for a QT tank.
If you end up having to medicate your fish in QT with copper or whatever, you can use old water from your DT to keep the water changes going for your QT. The copper will kill any ich that may be in the water column, and you will only have make water for one tank. New water goes into your DT, and old water from the DT feeds your QT. Please note: Do NOT treat your DT with copper, but treat the water coming from your DT with copper before adding it to your QT if you have to use copper...
Best of luck, and keep us posted.