How long has the pond been running? IE has it cycled. The fish nibble on algae, plants during the winter. If it is empty of plants etc you might be better to wait until spring. Winter can be hard on fish esp in a new pond.
One winter I decided to pull my lilies from the big pond, put them in a smaller pond, so the fish had a nice clean pond for the winter. When I put them back in early that spring/late winter the fish attacked the lily pots stripping every bit of algae within minutes. They were starving. I never tried to clean the pond for them again.