The garden pond's surface shouldn't ever totally freeze over because that would trap harmful gases beneath the ice. Toxic gases can escape and essential gas exchange can occur at the pond surface thanks to the EHEIM ice preventer, which keeps the pond surface from freezing over at temperatures as low as -20°C (-4˚F). This keeps the pond water's biological self-cleaning and detoxification intact while continuing to provide oxygen to the fish and microbes. An indestructible heater made of stainless steel keeps the pond's surface from freezing over at -20°C. keeps the pond's surface gas exchange vital. Constant provision of essential oxygen for microbes and fish The pond water's biological self-cleaning and detoxification processes don't change. Fermentation gases that are toxic may leak out. Strong, unbreakable heater manufactured