I am against aquarium heaters because
1.they are made to keep the water warm. Warm water will evaporate into the air of the coop much more than cool water, so it will increase humidity and also increase frostbite risk.
Also, creating warm water costs me way more money than simply keeping water from freezing.
2. Aquarium heaters will bust if run dry.
I greatly prefer a submersible bird bath or stock tank deicer (see, deicer... so keeps the water liquid,not warm). The one I have is safe to touch, safe on rubber, safe if run dry, and has a built in temperature switch, so it only runs when it NEEDS to run. If my power goes off and the water freezes solid, once I get the power back on, NOTHING is broken, and it is strong enough to thaw itself out of the ice block.