The textbook answer is, Feed normal food until the water temperature goes down to 10C (50F), then feed cold weather, wheat germ based food until the water temperature hits 5C (about 40F), stop feeding. In the spring start feeding cold weather food when the water warms up to 5C, then start normal food at 10C. Only feed as much as they'll eat in about 5 minutes.
Koi like treats too cabbage, oranges, watermelon are good, mealworms too, I don't know if goldfish like stuff like that.