Bent Tail

hi,
i have kept fish for quite a while including tropical, marine and now goldfish in a outdoor pond,

i have 6 goldfish, all are fine and eating well but one has a bent tail( ever since we got him it started to bend) and now its got to a point for a while where hes not as active as he used to be and struggles swimming as he has to move/ shake his whole body to swim, but i have been keeping a eye on him its now been 2 months and he is still swimming but not as much as the others and is still eating.

so got up this morningand the pond froze over apart from a small section where the fishs like to call 'home', ( the pond froze over due to a fuze blowing cutting out filter, pump and heater) after removing ice the fish where all fine apart from the one with a bent tail who is called 'bendy',bendy was not moving so we decided to move him into a tub indoors and let him recover and no he is fine again still slow, and he is now eating again but his backend seems to raise / float when he stops moving which i gather he may be a little constipated

so i was just wandering why is his tail bent, is it hurting him? when do you think i should put him back into the pond or shall i wait untill after the winter, what would you do ...

any comments, thoughts, stories, links ..... are welcome

here are some pics of him in the tub(not the greatest pics i have to admit, sorry)

In my opinion, (as it is in humans) you do get these odds and it is probably some inherent disability which will never be"cured".... If it does not restrict his being (quality of life),... Let it be...

A small electric shock could be possible, though it seems unlikely that just one fish suffered as a result. Ludwig's answer seems the most accurate and most deformed fish die off unable to cope with competition. I'd leave him indoors if its not to much trouble just so he can be in a better condition for when you release him back into the pond.