Years ago I had one of those hard shell pond inserts. I left it above ground. The kids put their carnival goldfish in it. I didn't know how many they put in and I didn't get it torn down before winter rolled around. We live in Western New York. Some days it gets very cold -5 to -10 degrees F. It froze solid. It was 1.5 feet deep. There is no question on whether it froze solid or not. No rocks or mud for the fish to hide in. The girls asked me how their fish could swim in ice. When the pond started to thaw, we saw dead fish. When the ice was all gone, there was two fish swimming around, and a frog. The frog could have been a new occupant, but the fish weren't. They survived the long winter in the ice. I don't know how they survived, but they did. There is no way that pond did not freeze solid in a Western New York winter. Every year since, I've removed the fish and they have done great. The one was 12" when a raccoon finally got him last year.
