In one of my ponds, I use a small 1200 gph fountain pump, set on a milk crate so it's just about 8" below the surface, and have it shooting water towards the surface to help keep and opening. Then have a floating tank heater (if you have a farm store nearby, they are usually cheaper than if you go to a pond store and buy them, plus higher wattage, so keeps the water thawed better, although they cost more to run, too), over the moving water to disperse the warmth. In the koi pond, I have an aerator blowing bubbles to the surface. Again, the bubbler is tied to a milk crate, so it's not taking the bottom water and stirring it. The tank heater is above that area, too. And, both of these systems are just in front of my skimmers, which I keep running all winter, if possible, for more water movement, to help keep the ice from getting too deep and not allowing water to move into the skimmer.
Also, my bogs run all winter long, and maybe because the water circulates from deep in the bog, under ground level, they have no trouble running all winter, even last winter with the extreme cold we had. Supposed to be colder this winter. Ugh!