I use to work at a pet & feed store where we would carry koi seasonally, and I too wound up with a bargain butterfly koi (Charlie) well I already had an Orenji Hariwake who was a little bigger than him and a few little pretty comets, any who "someone" started nipping Charlie's tail and next thing I know half his tail is gone. It grew back there was some scarring, a little bump where it grew back. That wasn't the last time either, at one point I thought I was going lose him, the next time his whole tail was gone, yet again it grew back though this time it was wonked, very resilliant fish. they are fast healers if they are healthy and I hope you don't have any bullies like I do. I put Charlie in his own tank to heal, the tattered tail can prove to be a little too tempting for greedy fish, even if they are small. best to you

Oh I forgot! in case you didn't know koi tend to jump occasionally, we found that especially when they are new to an area, you know newly shipped or they've been disturbed for cleaning or what have you. at the store we covered our makeshift pond every night just to keep them safe, little worse than comming in to work to find a $200.00 fish dried and crusty on the floor.