Fish are commercially bred in ponds in warmer areas of the US, mainly Florida. Many folks around here will run outdoor tubs during the summer for breeding, I have a 150 gallon for breeding platys. I set it up way early last year, had a buddy cramped for tank space, and an offer of a lot of free fish. I wrapped the tub in moving blankets with some carefully placed ducktape (not really carefully actually). Had a 1" thick piece of styrofoam for a cover, and a 1,000w heater, with no temperature control.
It got down to 29F outside, that heater gave a 60F temperature increase, no problem. It also made my electric meter spin like a top. I was unplugging & plugging in the heater a few times daily, for almost a week, until the heater controller came in.
So, if you can manage 7-10 wpg of heat, with an insulated cover, you should be OK. I have a second 1,000w heater, but the controller only goes to 1,800w, so if it got too cold for one I could run 2, with one on the controller. I only have this setup running from spring until fall, the electric cost during the winter would kill me. Removing the cover when it is cooler out results in some cool looking steam, and it seems tropical fish don't mind snow hitting the water at all.
If you plan on running tropical setups outdoors all winter long buy some stock in your local electric company to help offset the cost of heating.