How do I protect koi fish in a backyard pond during winter?

Protect koi fish during winter by adjusting feeding as water temperatures drop below fifty degrees Fahrenheit, since fish metabolism slows. Stop feeding entirely when temperatures fall beneath forty degrees to avoid uneaten food and water pollution. Maintain steady oxygen levels by using aerators or pond de-icers that keep an opening in ice for gas exchange. Avoid sudden temperature changes and keep water clean for fish health through the dormant season to ensure survival until spring.