The purpose of a de-icer is to melt a hole in the ice to allow oxygen to enter and waste gases to escape. The Aquascape device has an indicator light which glows red when it is heating and blue when it isn't. I like this feature very much, but because of it I know that it starts to heat up when the temperature is in the 45 degree range. This is quite unnecessary, since there is no danger of icing . Oddly the device seems to work best at very low temperatures. The coldest we have been so far is 6 degrees, and at that temperature it melted a nice round hole in the ice about 1 foot across just like it's supposed to do. However at 27 degrees it melted a hole about 5 feet across! Not so good for my electric bill but at least I know the fish are safe.