Fish gills are extremely sensitive and can be susceptible to parasite, bacterial and fungal diseases.
Gill disease diagnosis Often the disease is untreatable by the time it is apparent that something is wrong as we cannot easily see what is happening. The usual early signs are fish respiring heavily, fish laying on the bottom for long periods - general lethargy - not eating, fish tending to use one pectoral fin, keeping the other folded back against the body. At a more advanced stage you may notice that the fish can't fully close the operculum because of gill swelling, affected fish may segregate and stay alone - often near the surface or water return and there may be strands of mucus trailing from the gills The other problem in advanced cases is treating against parasites. The combination of excess mucus and hyperplasia forms a secure shelter for any parasites between the secondary lamellae making them very difficult to get at! Treatments First make sure water parameters are perfect, if more than one fish is affected this will probably mean the cause is enviromental - do a major water change. This may do the trick in minor cases Examine the fish for parasites. At this stage a skin scrape from immediately behind the operculum will suffice. You could try a salt bath for two consecutive days if the disease is limited to one fish. I would also suggest treating the pond with Chloramine T to reduce bacterial levels. If these measures fail to help, contact an expert or Vet for further help. | 
