I have the same problem with a Kenyi.. she is in a protected containter in the 125 gal and in a few hours she is back to normal.. she is chased through out the tank and if she gets out, and does. I leave it tilted and she finds her way back inside.. Neat that they know where the safe place is.. She almost goes sideways to tail down as she is safe but the other pic on the container scaring her.. she goes back to normal in a few hours of being back in the container.. I dont' have a tank for her, so this works for now.. she is some what deformed with only hafe of a gill plate on one side and the others are taking her out.. Crule but necessary in a community tank...
Something I have noticed with a tormented fish:
Swim Bladder problems may not be the problem.. Some fish are tortured to death by others in the tank, as my kenyi is, and they go into a slumber/SHOCK like state for a while.. Most should do fine in a few hours IF THE injurys are not to bad.. Missing scales and niped fins should not kill, but the ordeal they go through is enough to "shock them into death".. I have lost some fish to nothing serious but the ordeal it self killed them... Natures WAY to protect the species in a injured/sick fish??
As you have him in a seperate tank, he should realiize he is safe and get it back together you could say.. IF not and he dies, then he was tormited to much by the others.. NOTHING U can do to make it better, the Melafix has calming effects and should make him/her more relaxed...
I lost some Kenyi this way.. to many males and the domanate male started it and the others in the tank finished him off before I could get him out... Survival of the fitest is the way of life in a tank.. ONCE they get injured/attacted by others, it is instinct of all to attack and kill.. I see this alot in my 125 gal tank.. To many Kenyi, mostly adult males... Two nice males but they are holding their own against the others and each other for now.. NO one wants them and I dont' have any spare tanks.. So live plays out in my 125gal...
