It is important to establish the cause and resolve any underlying problems before any treatments are used. First check ammonia, nitrite and pH as unacceptable levels can cause gill damage. The affected fish should be examined for signs of gill parasites and gill disease. This obviously requires a level of skill and experience and access to a microscope. Professional assistance should be sought if necessary.
Treatment depends on the cause of the problem.
See gill disease page for more details. These are usually typical signs of a bacterial infection. Depending on the severity of the problem, the treatment can range from antibacterial bath treatments, to topical treatments or antibiotics. As with virtually all fish disease it is important to make a full examination of both the fish and water to determine if there are any underlying or secondary problems. These must be resolved at the same time as treating the infection.