In the first place, don't treat these fish with ANYTHING - not at first.

Test your water. Make sure the pH, the Ammonia, Nitrite and Nitrate are all optimized for the fish you're keeping. In the case of pond fish and goldfish, a pH of seven-point-five or higher is optimal. Nitrogen (ammonia, nitrite) numbers should register as "traces" and the nitrate reading should be less than sixty parts-per-million.

If the water quality is good, you should seriously consider that the fish is being predatorized by a scavenger catfish. Plecostomus catfish are often employed as algae eaters in ponds and fish tanks. I have several in my pond. The drawback is encountered in smaller facilities as the fish grow larger and consume the majority of the algae in the facility, they seek out other sources of nutrition.

"If it was my Plecostomus, why aren't *all* the fish affected in some way?"

'Plecostomus attack' (Look it up!) is in some ways like a fly-strike. The Plecostomus will naturally "go after" the site they've opened. Not because they're trying to spare some fish - but because they are working an area and the nutrition is greatest where they have penetrated the scale barrier in the one fish - so they hound their victim until it's dead, rarely starting another case until necessary.

"I don't have a plecostomus"

Then consider a bacterial cause of the problem. Since this lesion is isolated to one fish and one side of said fish, it is reasonable to think perhaps the fish injured itself on a tight-squeeze or a sharp ornament and now a secondary infection has set in. This would be treated by swabbing the area gently ONE TIME with a cotton swab soaked in Hydrogen Peroxide 3% USP and then injecting or feeding an antibiotic. The reason you should only swab the lesion ONCE with the peroxide is because over-cleaning a wound will prevent it from healing. Injections of antibiotics are INCREDIBLY effective and are discussed in the Koivet.com and this web site. Feeding medicated foods is simple and relatively effective. The food is called MediKoi and is sold by PondRx.com

Adding MelaFix (Aquarium Pharmaceuticals Inc.) to the water can speed healing once the infection is under control.