We were having the same problem. The temp dropped to -5 to -9C and I was switching out and bringing the waterer inside to thaw. Finally decided to try putting a plastic bottle (750ml gatorade) filled with salt water into the waterer. It hasn't frozen since. Not even a thin layer of ice on the top. It actually melted what ice was left in the bucket. I know there are mixed reviews on this, some have said it doesn't work, but, so far, it is working well for us. We don't want to have to run electricity to the coop so it seems like a good alternative...so far. I used 1/4 cup of salt then filled bottle with water (warm to help salt dissolve), lid on and popped it in, just floats around.