Here in the Catskills of New York I shut the aerator down over the winter and bring it back on line slowly as the ice melts. You want to do it slowly as there will be some nasty smelling water down below that could overwhelm the upper layers of water and reduce the disolved oxygen to levels that may harm the fish. Once the smell goes away we run it 24X7 until winter sets in the next season. Have not had a fish kill using this method.