I'm so sorry
I worry about trusting just one thing ( what if it should fail ? ) so pick 2 or 3 "systems". I use a Pond Breather, my smaller aerator with 4 airstones placed about 10-12 inches into the water, a pond deicer if it turns really frigid ( it's already in the pond and I plug it in if it turns bitter cold ) and I cover my pond.
Certainly this is no guarantee that something could still happen, but at least it's not dependent on one "system".
Again, I'm so sorry you lost 2 of your beautiful butterfly koi, it was a hard winter for lots of us.