I seemingly have a recurrent problem with my Koi. The first time a 2 year old guy took ill, and 2nd day in quarantine his tail bent upwards. He never ate again and I had to put him down a month later.

Last spring I bought two (very expensive for my tastes) (but very pretty) Koi, and as one immediately jumped out of the pond I put the one I could catch inside in my aquarium, left the other in the pond (uncatchable!)

Anyway, the inside fishie started to swim weirdly, shaking his head instead of his tail. Over the winter his tail started to turn to one side so he could only swim in circles. I nursed him along as well as I could until his tail did almost a complete 180 turned back, and he took ill and was put down.

I just noticed the other fish in the pond now also was a bent to the side tail.

While I don't think all 3 had the same problem, what is with 2 fish from the same source that both develop this same sideways bent tail? Could it be something genetic or some other problem I should worry about?