With you pond, its size, and your low temps, choosing temperate fish is going to be necessary, unless you are willing and able to bring them in seasonally.
Trying to heat it will be very expensive, and haphazard.
There are snakeheads that are from temperate areas in Asia, but those specific types will need to be chosen.
I kept temperate cichlids in a pond in the US midwest, in a similar size pond, but they, (and even plants) had to be extensively researched and chosen very carefully for hardiness, and even then, the most hardy, had to be brought inside in winter.
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