My husband and I won/rescued a fish from our local fair.  He has nasty fin rot, red streaks on his fins, and some missing scales.  Fortunately, the fin rot hasn't reached his body.  He was in smelly water, his bag was probably running out of air from how he was gasping, and he was chewing on bits of his own disintigrating fin floating in the water.  We are going to try to save him, or at least let him end his days in comfort.
What we're doing so far:
- We put him in a one-gallon treatment tank with an airstone and treated water from our own fish tank.
- Added Pimafix to the water.
- Gave him a bit of goldfish granules.
- Put him in a shady spot.
- Gave him a hollow fish toy to hide in.
Here is a picture.  If you can think of anything else we can do to make him feel better, please post.

Very cute fish. I would also add that testing the water will help as well; monitoring the water quality will help him very much. 

Yep, that's right Nossie, about the peroxide...it helps a lot, trust me. To treat the fish that way, you must hold it in your hands gently (above water, in case it wriggles free) and dab the rotten areas gently with the peroxide. When you put the fish back in the water, sometimes you will see a cluster of bubbles forming on the infected area...a sign that it's working.

Good luck Nicole! 

I'll give you all karma!
As of today, he seems to be a bit more chipper.  He's still clamped on his top fin, but he's swimming well and eating.  I'll try your suggestions today.
Of course, the baby is curious about our new pet.  He's been watching the fish quite a bit and trying to lift the lid off.  He seems to know that fishy is sick, and tries to reach in and give it a hug.

Aww, Nicole, what a nice and caring son you have
Good to hear the little one is getting better too! I'm sure he/she'll be fine in no time!

Update:
We had been treating the fish for several days and changing the water every other day.  Yesterday, I noticed that the treatment tank was smelling very foul.  It was so bad that our entire living room stank.  I changed the new fish's water, then did an ammonia test.  The reading was off the scale, so my husband and I decided to move the fish to the main tank, as its fin rot seemed to be gone.  It seems to be fairly healthy, if a bit jumpy, and, other than a "test nibble" on its tail last night by Spot, seems to be getting along well with the others.
On a bad note, our 60 gallon tank is now officially overcrowded with six goldfish residing there.  The tests show the water is slightly acidic, but everything else is okay.  We plan on stopping by the pet store to pick up an extra bag of gravel and a new box of filter inserts before setting up the old 40 gallon tank.  We plan on moving in at least two of the fish in there when it's all cycled and ready to go.

Oh yeah, I know the "Poor fish! You have to move in to the main tank!"-thing xD Well, glad everyone are doing so well for you
As Ireiden said, please keep us updated!

What a relief So gal it all worked out