I have a mesh cover and the water mostly goes through. When it gets very cold and the water in the mesh freezes, it sets up a chain-reaction where the top becomes non-porous and the ice will build up. A pillow may help as it puts the weight directly down on to the water instead of pulling in the side walls. You can use old truck inner-tubes too, if you can get them, and save some $$$. My father would use about 8 of them in his pool instead of pillows.