I live in NH, and it absolutely freezes ALOT here. 1 of my wagerers is 5 gallon bucket, with a 125 watt heater and it has 5 horizontal nipples that have never once frozen. In years! I've had plenty of regular cups and automatic cups freeze a little, but none of my nipples. If they start icing...
No to aquarium heaters in that kinda set up. Just go on Amazon and there's a ton for exactly what you're doing. Aquarium heaters are not designed for what you're doing. Just asking for failure. I live in NH and I run a 250 watt donut looking heater in my 55 gallon barrel and recirculating pump...