just chill out for one, that thing on the tail doesnt look serious, jsut observe it for awhile, its not gonig to kill your fish i can say that with confidence.
and dont add meds if you dont know what the heck your treating.
Your aro is just fine. It looks natural too me. I've owned a few of them in the past year and am currently keeping 2. That bump just seems to be a natural inclusion to these fish.