I assume based on your temps you live in the Tuscumbia in Missouri, not Alabama.

I am in Kansas. My previous neighbors used to have two koi ponds and never suffered a winter kill (that I know of) in over 10 years.

My kids used to go over and feed the koi, but I never closely examined their systems. However, I believe both of their ponds were about six feet deep. I think that helped the fish get through the winter.

I know they deepened the smaller pond one time when they still lived there, but that was due to a trash panda (raccoon) incident.

Hopefully, you can get an expert to respond and keep your goldfish happy!