Fin rot kills a lot of goldfish unfortunately. Everyone experiences having fish with this problem occasionally, but in properly cycled tanks with good water conditions it should be a very rare occurrence. Sue from Illinois posted some advice she got to the Goldfish Mailing List. This comes from Jo Ann Burke, the proprietor of Dandy Orandas in Alabama. Jo Ann has a lot of experience and has spent time taking classes on fish health at the University of Florida, I believe. So I was happy that Sue posted this information, which I have edited and offer here. Note that this is not an endorsement; please consider this informational (the unitalicized notes are my comments).
The advice is to:
The result: In two days the fish was better. In 7 days all the redness was gone and he went back in the big tank.
Let me add my observations to this. Both bacterial and fungal fin rots are secondary infections; the contagions are always present and the fish will only get these diseases if they are subjected to something that weakens them. Water quality, including chemical contaminants, and physical injuries are among the common problems.
From my experience, if you remove the cause of the fish's stress, the fin rot will quickly clear. I put a fish with fin rot into a 10 gallon quarantine tank with a cycled filter and one quarter cup of salt (about 2 teaspoons per gallon). The disease stopped progressing within 24 hours, and the fish's fins were growing back within a week.
I've also tried the antibiotic Furanace to treat bacterial fin rot. Seemed to work well, but it makes an unholy mess; sticks to everything. Ended up stripping down the tank, scrubbing it out and throwing away all the filter materials.
Treating the infected areas directly seems like a good idea. I have not tried an antibacterial cream. Many people recommend cleaning the infected areas with hydrogen peroxide and then applying mercurochrome, though this hasn't been effective for me. Silver nitrate might also work. Just make sure you don't get ANY of this stuff in the gills or eyes of the fish.
To handle the fish for topical treatments, place it on a wet wash cloth, fold this over the body, and then use firm but light hand pressure to hold the fish in place. Then basically follow JoAnn's advice. I recommend wearing protective glasses, as one good tail flop will put whatever you are applying all over you. It is helpful to have an assistant handing you appropriately dipped cotton swabs.
You can keep the fish out of water for a minute with no apparent damage, though they don't enjoy it.