Are you feeding the fish ? Have you had the pond electrofished ? You need to be sure of the situation first. See these from the archives.

http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=255372#Post255372

http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=92492#Post92492