This is a tidbit I sent out to my local club:

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Now is the time to go out and get a heavy duty remote controlled power
switch. Easy to find as they are usually in the area with all these colored
lights, ornaments & shiny things this time of year. Why do you need one?

If you are using any sort of stock tank heater they do not have the best
thermostatic control and some will actually continue running after the pond
has reached 45F. With a remote you can turn the heater on and off as needed
without leaving the house. I do highly recommend some sort of lighted
ornament (hey, these are cheap and plentiful right now too) so you can
easily see that you've turned on/off the heater. Plug this into the remote
using one of those dual or triple plugs (also plentiful and in the same
area).

Personally I use one of those large red cane/hook looking things I got at
Wally World for about $2.50. ;-) Since I have my pond on slow flow, I like
to add a little heat on those really cold nights.
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Our Koi Club Newsletter for Nov/December is now on-line with some great
pictures: www.midcolumbiakoi.org ~ jan
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Zone 7a, SE Washington State
Ponds: www.jjspond.us