I've had situations similar to your's. My pond had so many water weeds and duckweed or algae was thick enough to block out the light so every time I had several cloudy days in a row my fish would stop eating and would be gasping air in the morning.
I would set up a pump and it always made a huge difference. My pond is smaller than yours but the pump put a stream of water a foot into the air out of a 1 1/4" pipe and the next day they weren't at the top and two days later they would come to eat. I always ran mine till the sun came out.
I've since cleaned out the pond and thanks to Pond Boss I know what to watch for and I manage the weeds better. My diffuser quit last fall and I've gone without this whole year but I have electricity there for emergency.
Good luck
