Almost always a fluke infestation.

Some rare exceptions would be bacterial gill disease, and the two conditions are often super-imposed. Bacterial gill disease is recognizable grossly in moribund specimens because the gills are basically red looking with patches of rotten tissue. Sometimes, more subtly, there will be streaks through the gill tissue, rather than around the arch. 

If you see signs referable to Bacterial Gill Disease you should seek professional help from a qualified person, and in the meantime you can treat the infected fish in a bowl with Chloramine-T at the rate of 1.5 grams per gallon of pond water for one or two hours - but again please ensure that you do insert an air stone into the bowl to provide oxygen and good circulation of the water .. If the fish shows any signs of stress then remove from the treatment bowl and return to the pond.