Here's the info:
I do not know them Ammonia, Nitrite, Nitrate, or Ph Levels. Sorry...
25 gal tank running for over a year.
Eclipse 3 biowheel filter, 250 gph.
25% water change monthly.
Three goldfish, all about 3.5 inches.
Melefix to help regrow some frayed fins, and Proquatics Bacteria Starter every week.
No new fish in over six months.
Feeding Bio-Blend Goldfish Food.
Had fungal infection about 2 months ago, but was treated and have been doing fine.
One fish has two or three visible veins in his tail (not bloody fins).
No unusual behavior from the healthy two, but the sick one had been kinda rolling aroung occasionaly.

Okay, so here is my problem. I was treating the tank with PimaFix and Melafix for some fungal and fin problems. That was about a two months ago and all is well, or so I thought. I noticed last night that my Ryukin Goldfish, Cocoa's left gill is swollen and puffy, but only the left side gill. It is still red and healthy looking underneath--but the outside flap, for lack of a better word, is puffy and looks like a balloon filled with water. There do not appear to be any parasites to the naked eye, nor any other visible symptoms. I also noticed last night that he/she is acting odd from time to time. When I feed them, she kinda rolled around like she was dizzy, but then a few minutes latter she was fine and swimming like normal. What is going on? Any help would be very appreciated.

It sounds like the gill flap itself is infected but that doesn't explain the rolling around behaviour.

The best thing I could suggest is to feed some medicated food such as Medigold which is available on-line from Goldfish Connection.

For the rolling behaviour try feeding your fish peas only for a couple of days. Cook the peas until soft then squish them until they pop out of their shells then break them into bite sized pieces. Around 2 peas each per day.

You could also try changing 25% of your water weekly rather than monthly. This will help prevent those pesky problems like finrot and fungus.