Aquarium heater in pond

I have read of people's varying experience with putting a submersible aquarium
heater in an outdoor pond over the winter. I think Ingrid did it succesfully.
Can anyone provide personal experiences, what to do and not to do? I am trying
to keep a lotus alive in an above-ground stock tank in Chicago that is too
heavy (read I am too lazy) to move and store, and want to put in a heater and
coer the whole thing with white foam insulation board. I'm not worried about
the rosy reds and feeder goldfish that were in there to eat mosquitoes, but
want to prevent solid freezing.
Thanks!