I'm in NY and back when I had a pond, my fish never had any problems surviving winter. My deep end was I think 3.5 feet. I never used heaters or bubbler to keep a hole in the ice. When the water temp starts cooling in the fall, I would stop feeding the fish and remove all plants. In the deep end I put in a clay flower pot that had hollow bamboo or stiff reeds or anything like that. Purpose is to keep surface from freezing and allowing bad gasses to escape. The fish go dormant and no problems. I had comets and shebunkin (Sp-sorry).