Apologies if this is in the wrong section but wasn't sure where to post for Golfish kept in tank.

I have a fancy goldfish that has been attacked by other fish, it has virtually no fins or tail left, I have moved it to a tank on it's own and it's hanging on, but can't swim very well, ( obviously ) and is upside down most of the time.

Could anyone tell me if the fins and tail will grow back at all or would it be better to put it out of it's misery?

Many thanks