I wouldn't worry about cold temps. I live in Vancouver and had a 3'x6'x4'd pond for well over 20 years. Our frost depth is 18" and in the winter the pond would freeze over. I kept a ball in the pond so when it froze just push the ball to break the ice. Fish metabolism slows down. I started out with little feeder fish like yours and they grew to well over 6". I rarely fed them as they got lots of bugs and stuff. I also went to the local lake and got some indigenous water lily which took over the pond and came back every year. Finally some racoons moved into the neighborhood and although the fish had lots of protection I got tired of finding my pond trashed as the racoons tried to get at the fish. Now it's a nice paved patio.