The problem with a trough heater is that it will eventually fail. My
first one lasted five years. It was a 10 inch disk that looked like an
anti-tank mine. At 1200 watts it kept a one foot wide tunnel open to
the air. The depth of the ice depends on how much snow we get in
November. More snow means more insulation and less ice so a foot or so
of foam might work.

After five years the heater became waterlogged and wouldn't float
anymore. Being the inventive fellow I am I propped it up on concrete
blocks. But this didn't allow it to follow the level of the pond and
when it dropped the heater was exposed to the air and burned out. End
of fish.

With temps to -30F I prefer 1200 watts, but you might get by with
less. Your pond is so small that you could buy a couple of 4 by 8
sheets of foam, glue them together, drill a hole (6"?) or holes to
allow air exchange and this would give you a jump start on the
insulation provided by snow. My heater doesn't run that much because
we usually get a couple feet of snow early in the season and that's
great insulation.

Still, the volume of your pond doesn't give you much flexibility on O2
content, so I'd be concerned if the fish are valuable.

My city floats firehose sized pumps in some of the lakes and runs them
all winter to prevent a fish kill.

Good luck. Let us know what you end up doing and how it turns out.

dss