Damaged tail - any ideas?

One of my sarasa comets, Hector, seems to be suffering
persistent damage to his tail, and I'm anxious to identify the cause, so
I'd appreciate any suggestions. At first I thought it was splitting as
he was growing, since I've seen that happen, but it hasn't healed up as
I would have expected in such a case, and it's been going on for months.
It's now looking ragged rather than just split. The other obvious
possibility is biting, but he socialises well with the other fish in the
tank (another comet, a blue orfe, two koi and three weather loaches) and
there's no evidence of bullying. Neither is there anything sharp which
he could be catching his tail on.
Naturally, I've wondered about fin rot, but none of his other
fins are affected (they're all in good condition), and I would have
expected fin rot, unchecked, to be fatal within a few days, not to
persist for months. Do any of you know of any kind of slow-acting
infection which might be to blame? If so, how should I go about treating
it?

Jennie

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Jennie Kermode
http://www.triffid.demon.co.uk/jennie