Bacterial infections are usually "secondary" to some other primary cause. With tangs, sometimes they injure themselves when the wedge into or against rocks to sleep at night. Also see this from shipping injury as the fish rub up against the bags, etc. while traveling. Very common to see infections pop up in the areas like those on your fish. But usually the fish's immune system can handle the scrape/injury. Need to boost your fish's immune system. Adding vitamins to the food is a good step. Red nori has a bit more vitamins than the green varieties. To the other foods adding vitamins like selcon, zoecon, vitachem are good, but I'm really liking adding Beta Glucan to my fish food mix. It tremendously boosts the fish's immune system. Highly recommend BG.
For infections in the head/face area have had very good results with Sulfaplex (aka Triple Sulfa) + Neoplex. For eye infections, erythromycin is the best. Remember antibiotics don't cure anything. They just help give your fish's immune system "time" to fight off the infection and heal. So great water quality, excellent high quality foods, and low stress environment will all work together to help your tang to heal itself.
