I live in Northern Virginia. I have a 800 gallon open stock tank that is all above ground. Even with all the really cold days with early mornings less than 20F and several days not getting above freezing, my tank hasn't frozen solid. I still see my goldfish in there. This is my second winter with the tank. I have a 1200 watt floating deicer and I have had up to a half inch of ice on half the tank. I also have an air powered filter running. Last winter the tank froze over the deicer making the only opening. Being 8 feet wide, I think the ground underneath doesn't get cold enough to freeze the water from underneath.
If I get a chance, for next winter, I'll try to build up a wall around the tank to provide some insulation and make it look nicer.