You have to be careful feeding goldfish once the temp drops below 10C as their digestive systems shut down (only partially in warmer areas) and they can have constipation and other problems as their systems can't process the food as normal. I stopped feeding our pond fish a couple of months ago.
If they can, put the fish in a small tank and bring the temp up in it. The fish should come right again. Once it has, put it back in the pond. Just a tank inside a warm house would do, no need for a heaterstat.
A cooked, shelled pea would help it's digestion too.