I found this disease:
Columnaris
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A bacterial disease caused by the bacteria Flexibacter columnaris. This disease is usually associated with some kind of stress condition such as high water temperature, low dissolved oxygen concentration, crowding, or handling. Symptoms of this disease include grayish-white spots on some part of the head, fins, gills, or body usually surrounded by an area with a reddish tinge. The columnaris lesions on different species of fish vary in size, location, and appearance. On fingerling rainbow trout, a lesion usually originates on the back of the fish and progresses down each side resembling a saddle. On crappies, the lesions are generally confined to the fins and gills and rarely extend to the body. The lesions on bullheads generally appear as small circular areas with sharp distinct outlines. Although columnaris most commonly involves external infections it can occur as an internal systemic infection with no visible external signs. Scrapings from a columnaris lesion placed under a microscope will reveal long, thin, rod shaped motile bacteria. The bacterial clumps form microscopic columns or dome shaped masses, hence the name columnaris.
Says it is surrounded by an area with a reddish tinge, and can attack just the gills. Only other gill disease I know is gill flukes.
