why are the fins tattered up? Normally you dont get sudden scale loss for no reason. Seems like its was bullied before and now its getting an infection or reaction to it. Heat, mela and pima fix and some water changes should heal him up.
Yes I had a small black aro get the crap beat out of it by two other black aros.
You need to separate the fish, treat daily with melafix and pimafix and daily 50% water changes. You'll be fine as long as it is still eating. If it doesn't eat then likely has internal injuries and little if no chance to survive.