Gerri, It doesn't happen often, so far twice I remember in my soon to be 68 years, but temperature in north MS have hit down around -15 def. F with no real damage to fish in farm ponds, most of which are no deeper than that. It only last a week or 10 days as best I remember so longer might be a problem. I was surprised during my winter visit to MN to learn ponds up there freeze solid in the winter.