Koi and goldfish do not need to eat when water temperatures drop below 55 degrees. That’s because they are cold-blooded, which means their body temperatures matches that of the water temperature. Cold water = slow metabolism = decreased ability to digest food.
Undigested food either backs up in their digestive system (bad for the fish) or rots in your pond (bad for your water, and, consequently, bad for your fish).
Start feeding your fish cold-water food when the temperature starts to hover between 50 and 60 degrees. This food is easier to digest than regular food and helps ease their systems into winter mode. As the temperature drops, watch your fish for signs of decreased activity. If they seem to be slowing down, and if your water is consistently below that 55-degree mark, you’ll know it’s time to start gradually decreasing the amount of food you give them.