Listing Goldfish

Hi all,

I'm having a problem with a goldfish I hope you can help me with.
First off, it's a very small, 5'x7'x2" deep backyard pond with a liner.
I've
had it about 5 years.
The fish in it are all garden variety goldfish, ranging in size between
1.5"
up to this one, who's about 6". The pond is a little overcrowded, but the
water chemistry checks out, I'm using a bio-filter with charcoal, in
addition to a watefall whcih helps keep things aerated.
All the other fish seem fine and happy.
The water is dropping to around 50F, so I've tapered off the feedings,
which
have been fall and winter food for about the last month.
There have been no signs of parasites; but I did treat a week ago with
permanganate. I usually do that at least in early spring and late fall,
just
as a precaution.
Now to my troubled fish.
He's a goldfish, about 6" or 8" long.
I would guess he's about 3 years old, born in the pond.
For the last few weeks, he's seemed very sluggish, not swimming or eating
much, hanging out at the plant shelf, which is about 10" deep. As I said,
the other fish seem happy and frisky.
Today, I changed the charcoal filter in the bio filter, and he actually
seemed to perk up, and was swimming about a bit. Problem is, he seems to
be
having "attitude" control problems. As in, he was nearly vertical at one
point, then leveled out, swam some more, and then drifted back to nearly
vertical. Then he swam away, and acted as if nothing was wrong.
The only thing I can physically see on him is that his scales look maybe,
for want of a better word, unkempt. A little rought and un-lustrous,
maybe.

Any ideas? Do these guys just not live any longer, or is there something
else wrong? As I said, I've seen no other sigsn of anything wring in any
of
the other fish.
The nitrites, Ph, and Ammonia levels in the pond all look good.

Thanks,

Sean