In deep winter, as fish enter torpor, you'll need to stop feeding them altogether. Fish are cold-blooded animals and their slowed metabolism is unable to properly digest food. As temperatures begin to cool down, many pond owners choose to change their fish's food to an easy-to-digest wheat-based formulation, rather than one high in protein. This isn't strictly necessary, though, since a healthy well-nourished fish can easily last through a winter of extremely low activity. As weather cools, you'll want to invest in a good water temperature meter. Once the water temperature (not air) drops below 50 degrees Farenheit, it's time to stop feeding until things warm back up in the spring. Once the water temperature climbs to 50 degrees again, it's fine to resume your normal feeding schedule.
Fish don't actually hibernate, so you may see them moving slowly around from time to time. This is when you may be tempted to throw them a bite here and there but stay strong! Slowed metabolic activity can leave food that's not completely digested, and it can get lodged in their system and kill them. It's far better to err on the side of caution. Even if the weather should temporarily warm up and your fish become more active, keep to your plan. Your fish won't starve as their bodies are meant to go through a dormant winter cycle.
