This is not an actual single disease but rather a symptom caused by a range of different diseases affecting the gills. It may be caused by chlorine from tap water, ammonia, fungal infection, bacterial infection, or parasitic infection.
Symptoms of gill disease
When you see your fish opening their gill covers rapidly, have swollen gills or discolouration of their gill filaments then this is a sure sign of gill disease. Fish will stop eating or lie at the surface of the water gasping for air.
Cause of gill disease
Newly imported fish kept in poor conditions or fish kept in poor conditions such as tanks or ponds that have been neglected. Using untreated tap water, poor filtration, overcrowding, lack of partial water changes or rotting food waste can all cause gill problems.
Treatment of gill disease
Check water conditions. Check for chlorine, chloramine, ammonia, nitrites and nitrate levels. Add an antibacterial medication to the water. Put affected fish in a hospital tank. Do an immediate 25-50% water change with new water that is aged or treated with anti-chlorine mediation.
If no improvement is seen after an hour after the water change then you have to suspect gill parasites. Treat the fish with an organophosphorous insecticide such as metriphonate, formalin or copper salts.
However, gill fungus is a very difficult condition to treat. This should be suspected when the water change and the parasite treatment has failed. To treat gill fungus you keep good water conditions for weeks while treating with an antifungal medication to see a slow and gradual improvement in the condition. You might see some fish losses until then.