Frito,
If you placed some hay or straw bales around your tank, that would help insulate it during the low temp months. Another idea (a little more pleasent appearance wise) would be some 4" styrofoam panels or blocks. Then it won't cost so much to operate a stock tank deicer / heater if you wanted one. It shouldn't kick on all that often (if it is a thermostatically controlled unit). Since you generally don't get below 20°, that's not too bad.
However, I am wondering if an aerator alone would be sufficient to keep the water open. Do you currently have an aeration system in your tank? If you do, that might be all you need. The fish might slow down, but they won't freeze. The aerator, placed appropriately, will also help circulate the water for you.
Out in western Nebraska sandhills country, my uncle had a cattle ranch. It gets COLD out there in winter. He had a metal stock tank out in the middle of a pasture, miles from any electrical lines. It wasn't insulated in any way, nor heated. But, he did have a windmill that pumped ground water into it as long as the wind blew. It never froze because the wind always blew out there and therefore, 54°F water was always being pumped into the tank. He just let it overflow and run down the hill.
Gordy