1) WARNING! DO NOT PLACE A FLOATING HEATER IN A PLASTIC WATERING TROUGH!
I refer specifically to floating heaters where the electric heating element is exposed (without a cold temperature cage) below the water line. If and when the heating element touches the side of the trough, you will have BIG problems.
My story of the day the watering trough burned to the ground is not an exaggeration!
2) Stupid of me not to think of pipe insulation as one possible aid in this effort (we use a lot of it at work on Liquid Nitrogen lines).
3) I personally have had a bad experience with heat tape, which melted it's own insulation and shorted out. It is entirely possible that this fiasco was the result of my own mistakes in installation. Regardless, Bruce, I suggest you set up, adjust, and monitor whatever powered heating system you use over a long a time as possible (weekend at the farm?) before leaving it to it's own devices. A bright guy like you is probably already figuring on doing that.