6,000 gph is a good flow rate for that size pond. You do not state the level of biofiltration.
I would try to remove as much of the organic debris as possible.
This being a northern climate pond, fish health issues are not unusual after the Spring thaw. During the winter the fish's metabolism is quite low which compromises their immune system. This makes them an easy target for any pathogenic organism that may be in the pond. It is practically impossible to control as the actual infection occurs during ice-over.
Did you experience ice-over this past winter and, if so, did you maintain an opening in the ice by means of a heater or aeration?
"Everybody is ignorant, only on different subjects." - Will Rogers