Found this on the net:
Question:
one of our koi has had a bent tail for a few years now but this season once the ice melted off the pond the tail is bend all the way sideways? what causes this?
Answer:
I'm sorry about your koi. Bent tails occur pretty frequently. Some fish are born that way. If not, then it can result from physical injury, vitamin deficiency, electrocution (such as a shorted out pump in the pond or even lightning), or fish tuberculosis (which usually comes with other symptoms like either anorexia or tumors).
and:
Why is My Fish Bent?
Here are some reasons a fish may develop a bent spine.
* If born that way, the fish may have a genetic abnormality.
* If born that way, the fish may have incurred a developmental problem while growing inside the egg. This could be related to diseases, temperature, injury, etc.
* If the fish was shocked via a faulty heater, pump, etc. or via lighting (striking a pond or surging through a house), this can result in a bent spine.
* A severe injury can bend the spine. Normally though if broken, the fish would die right away.
* Fish tuberculosis sometimes results in a gradual development of the curvature of the spine. I have seen this often in zebra danios but also in a golden orfe and a few other species. A fish with this condition (fish tuberculosis with a bent spine) is often also anorexic.
* Certain vitamin deficiences can result in spine curvature.
* It seems that many fish nearing the ends of their lives get bent spines which usually match one of the above problems but sometimes do not.
I'm afraid it can't be undone but the koi can still live a pretty normal life if he can still swim around.