So long as a sliver of ice free space is around the deicer, it is doing the job it is supposed to do. It is impossible to predict what the ledge will do. A bubbler would do the job too so long as the bubbles don't hit the deicer directly.

I don't get quite the winter cold you do, but the first winter I turned off the pump and lost most of my fish after the top froze completely over. Since then I just run my submersible pump with a prefilter on the inlet side but an open outlet, and it keeps an open space on the surface, as well as providing circulation. The ice has gotten thick elsewhere on the pond, and one winter there was even briefly a dome of ice above the pump outlet, but the pond stayed aerated and the fish survived. In combination with the deicer I'm sure it would work for colder zones.

We were having lows in the single digits, averages in the teens and rarely highs above freezing. The Pond had three inches of ice and I had to leave town with a forecast of no change. Out of frustration I just tossed a gutter/pipe protector (12 foot wire thing with a thermostat that plugs into the 110 outlet.) into the pond and returned after a week. Above the heat, I tossed it in coiled, was a two foot hole that was ice clear.

Hey how ya guys doing. I live in Pennslyvania, this winter has been pretty nasty. You either want a air bubbler or a submersable pump running that will spray water up. Thus breaking the ponds surface and providing oxygen for fish. I see you said something about a plastic ledge. Just make sure your pond does not freeze completely. Also, make sure that if you do have plastic over a pond with fish. Make sure it's not air tight so that harmful gasses can escape. I keep mine uncovered, I run a big underwaterpump that will spout water. This will freeze solid if it gets cold enough. I also have a floating pond de-icer that will keep a hole in the ice if everything freezes. Hope this helps

Craig