I dropped a bird-bath de-icer into a bucket with nipples. When the temperatures were above 25 or so this worked fine, but then the temps dropped into the single digits and much of the water froze, including the nipples. The waterer was outdoors in the run. This winter I plan on using heated pet bowls in the coop.
mine dont freeze but i keep a heated dog dish in there too just in case
DO NOT USE AQUARIUM HEATERS!!!!!!!!!!!!!! i dont mean to offend anyone that does but they are prone to overheat and break causeing shock or glass can explode if level is too low . i have tried to use them and did some triall runs with 3 different ones last winter all ended badly !!!