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I've had plenty of experience with aquarium heaters to know this is a dumb idea--the water level will fall below the heating element causing it to overheat and break which will not only drop pieces of glass in the water but short circuit and electrically charge the water container. Unless you're interested in electrocuting your birds or yourself, stick to heaters meant for chicken or bird waterers!!
Adding to the advice being given: I give liquid water to my birds inside a nonheated but insulated coop all winter. The temperatures inside often falls below zero and even using galvanized chicken waterer heaters under my double-walled waterers, I have had ice form in them. That being said, in 25 years of keeping birds I have never had problems with frozen combs or feet. While I have not kept large combed birds, i.e. leghorns, I have kept sexlink varieties and EE's. Most of the former have fairly good sized combs. If and when the birds get out in the snow, they will peck at it but I hardly think they use it as a liquid source. They also like to peck at snow that comes in on my boots when I enter the coop as well but given that they have water, I doubt this is for liquid just for fun.
