its definitly not fin rot, thats caused from a fish not having a big enough environment to move about, and you can put a damsel in any size tank pretty much. yea definitly STAY AWAY FROM COPPER SULFATE. he probly has ich just get rid of them, they are terrorist anyway. have your water tested at your local fish place. if its ok, get rid of all of them, and if its not, keep them until its good except for that ich infested one, take him out. i assume you dont have a quranteen tank?