First off, Welcome to TRT, from one Hoosier Reefer to another!(where ya from?)
Also , a picture would help a lot of the tang,,,,
Yup the trigger could have inflicted the damage to the tang,,
not sure right off what to tell you to use to treat it tho,,,
Yes if you could move the tang to a smaller
quarantine tank, the would be better, due the smaller size (less meds need to treat) and to give the tang a breather from possible future attacks,,
Was this tank a "got it as a package deal " from someone else?
and just re-set it up ?
reason i'm asking is, even in a 55 g tank, you still want to start off slow on fish,
You need to let the tank cycle/mature a bit b4 adding more bio-load in it.
a 55 will give you more leeway than a smaller tank will,,but you still want to take it slow regardless
maybe a
LFS owner like Jenn with chime in on what meds to go with
(or some of our fishy "doctors" here

)
Again, where you from in Indiana? maybe i can steer you to some good LFS(local fish shop) to check into for personal help,,,
Good luck , and again WELCOME TO TRT!