What are your water tests showing for levels? pH, ammonia, nitrite, nitrate, and water temp. A photo of the gills would also help if possible.
Also, are you giving the tank plenty of air? This would be unrelated to something behind or in the gills, but it might help it breath a little better if you get more oxygen into the water.
I'm not sure I can help much, but getting that information up will give you a better chance of someone recognizing the problem.