I will use the heaters if/when it gets cold enough that the pumps cannot keep up with the ice. This evening, both ponds and the stream look the same, so the pumps are doing their job.
Colleen, the bottom pump in the koi pond I rerouted for winter to run the bog, thinking with more water movement (4200 gph vs. the 1600 gph used previous winters) would keep the ice at bay. Don't think that worked either, it appears solid ice on the bog, although it COULD be running underneath. I have the exact same pump in the goldfish pond running a 3X larger bog, and it is running well, and did all last winter. I think the key must be larger bog, and maybe the goldfish pond bog is deeper, too, thus, brings in warmer temps down deeper in the ground. I never got the koi pond bog made larger this past summer, just could not figure out how to do it on that side, so left it to ponder for year another year. But, as you said, Colleen, the much smaller 1600 gph pump sitting on an angle held just bout 2' below the water surface is doing a fantastic job so far of keeping a nice opening in the ice. Same this evening as it was yesterday, and temps this morning were in single digits. Going to get even colder, though, so probably will have to break out the heaters for a few days. We shall see...
Since my daughter taught me in UT, they don't always use tank heaters for the horse troughs, since they don't have access to electricity where she used to board her horses, I have been chopping the ice and scooping out the chunks twice a day, and so far so good in that endeavor, too. I'm trying to keep from using heaters unless absolutely necessary. They cost me about $250 in one month last winter! Yikes.