Iwishtofish,
IME/O, the ammonia concern appears to be an issue with your test kit, as the pic shows no signs of ammonia posioning.
Post a new pic....
Going forward, here is what I would do....
1. Do a 50 to 70% water change.
2. Run the biofilter continuously
3. Run the biofilter with carbon
4. Place one or two pieces of live rock in the tank
Run this set-up for a week allowing the tang to adjust to his new surroundings/environment. During this time, just observe the behavior of the tang.
I would then treat the tank with "General Cure" by API. This is a combination of Praziquantel & Metronidazole. This will eliminate external parasites and secondary bacterial infections. If you have Prazi & Metron separately, either treat separately at specified doses or combined at reduced levels (i.e. reduce by 50%, etc.).
After the treatment protocol, observe the tang for another week or so. I would probably do one more treatment followed by a couple of days of observation before releasing into the display (as long as no other symptom manifests).
The wound on the side looks like it is healing nice.
Good luck