have you ever left it uncovered and just run a large air stone or heater?
I did that for many years, this is only the 3rd or 4th time I've covered it. The reason I decided to try covering it was based on some science posted on a koi forum, discussing wind chill and evaporation. Since my pond is only 1700 gallons and has 4 adult koi, I decided to give it a try.
I've been pleased with the results. The pond doesn't freeze over and when I peek in on them, the koi are more active then when I left it uncovered. They're not moving swiftly, but they move around slowly, and before they remained under the granite slab in the deepest section of the pond.
It is also my OPINION, that larger koi, need supplemental O2 during the winter, ( at least in my relatively small pond ) that a deicer alone isn't sufficient. I add 4 airstones, distributed around the pond and also run a Pond Breather once the weather turns very cold.
I detest how ugly the set up is, but am pleased with the results. The first year I tried it we had an old blue solar cover and that's what we used. Last year hubby ordered a new cover while I was out of town...and it's blue, I would have preferred a clear cover to allow some sunlight. If we have a warm sunny spell during the winter, I peel back the cover mid afternoon, so they can enjoy some sunshine.
Sorry for the novel like response