That's almost always a sign of an advanced fluke infestation; and you should undertake the following two things:

(1) First of all Net the fish and carefully place the fish in a bowl where you can examine it, do an oral exam of mouth, roof of mouth looking for Ulcer, gills, looking for swelling, paleness, streaks of white around the edge of the gill or through the gill tissue, or any kind of foreign body like a piece of pine straw intertwined in the arches or perhaps even a small stone in the mouth.

(2) Take a Gill scrape with a coverslip and examine this under a microscope for Flukes.  If flukes are found then treat those flukes with a carefully measured out mixture of Malacite Green and Formalin.... remember that you will have to re-treat the pond again in 7 to 10 days when any eggs from the adult parasites that have been killed, hatch out ...