do you have a pic? how much of its tail? first thing you need to do is separate it into hospital tank and away from the fish that did that to it, raise temp to like 85 and make sure to keep water really clean, im not really that experienced so im sure someone with more experience will chime in but thats what you need to do in the mean time, i think alot of people on here have had sucess with helping fish heal by adding salt and melafix though i have no idea how much salt you should add especially with a sensitive fish like the BGK
GOOD LUCK