I have used nothing but a breather for my fish filled ponds. The smaller pond 1000 gallons 2.5 feet deep, the other 8-9k gallons 5.5 feet deep, the hot tub pond around 2.5 feet deep about 300ish gallons, it is above ground. So far 4 or 5 winter without any fish loss. Can't recall how long I have been using them now.
I won't use a aerator after a friend loss every fish using one. We think the water was super chilled, it was a strong one.
Thanks for the information. I used 1 last year in my pond that was to shallow and the breather had a meltdown and failed. Fortunately I only lost 2 of the 52 goldfish. This year I now have a larger pond but also have some larger Koi 12+". I will install 2 breathers in case 1 fails while I am away for 4-5 months. Hope they winter ok. I think I will also cover part of the pond with plywood to stop the snow sliding off the roof from hitting the pond/ice. ( note to self to conect the 2 breathers to 2 different electrical curcuts for additional security ).