Nope, Ive looked closely for signs of disease ( Ive always had tropical fish prior to moving and Ive read quiet a lot of books about fish diseases and keeping tropical fish) I know nothing about coldwater fish and ponds. I think it could be weather related.
Now if you had a spare tank or vessal and mature filter, I would have suggested removing the fish and quarenting it indoors or a shed or somewhere warmer and adding some aquarium salt as a tonic, but I gather you dont have this
Is the pond completly iced over if so you need to break it and take any dead plants and foilage out and leave the pump on all night and try to keep the pond ice free as well, or put a a tennis ball on the water and it will keep a hole in the ice. Hope this is useful
I wouyld say 80-85% is iced over, Ive started to leave the pump running 24 hrs and the only part that isnt iced over is the part were the water is falling back into the pond. I visited Sheffield Aquatics today and asked them about it, he said as long as there is a small hole then it should be fine, he said people tend to put a football or some kind of pond heater on there pond around OCT time. The trouble is we are getting fine weather during the day but its not staying on the back garden long enough to melt the ice. I have a net over the pond too so the recent snow we had is covering the pond.