I have two outdoor ponds with Koi and have never used a pond heater. Usually normal winter for us means the ponds are completely covered in ice for 2 months so all I use is a floating pond de-icer in each one. What these do is keep a small hole open in the ice to allow noxious gasses such as carbon dioxide out of the pond. Of course all pumps for filters and waterfalls are turned off, over 10 years I have maybe lost 2 or 3 Koi to overwintering in the ponds.
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