Koi has lost its tail
A week ago we bought a baby koi (platinum ogon), about 5cm in size. We quarantined it and added it to the pond on Thursday. It seemed to be doing well, swimming around and exploring. It didn't take to the rest of the koi in the pond but I suppose that it's because they're much bigger; the smallest of that lot must be about 10 to 12 cm.
I came home this afternoon to find it at the bottom, hiding under the lily bag. After a while I checked back to find it swimming in its usual spot near the pump pipe.
Its tail is completely out and I'm not sure what could have caused it to lose its tail. Could it be that it had a run in with the bigger koi; or with the pump?
I've put it back in the quarantine tank. I'm really worried as, although its swimming, it seems to sometimes lose its balance swimming almost vertically head down and sometimes listing to the side.
What can I do to help this little one recover? Will its tail grow back?
Also what can I feed it? My koi dealer told me that goldfish flakes can be used instead of Tetra pellets. But when I put the food in it doesn't swarm to the surface like our other Koi do when they're fed.
Any help will be greatly appreciated