This is my first year trying to overwinter Koi in my pond. I am sure they did not make it through the winter. I used an air stone to keep a hole in the ice open. Ammonia stayed at Zero, General hardness at 60, carbonate hardness at 120, PH at 7.0, No2 and No3 at zero. 4 feet deep pond. 8 feet wide, 6 foot long, 4 Koi that are less than 10" and three 5" goldfish. Zone 6A. The pond never froze any deeper than 8 inches in any area away from the ice dome from the 8" hole near the bubbler. If my fish died during my first attempt, I will probably never try to over-winter koi again.