I have my pump running about 9" below the surface on one of the preformed
plant ledges, that way the water on the bottom doesn't get circulated and
the pond doesn't freeze over. I talked to the owner of a local petstore and
he has a pond the same size as mine and he said that is what he does and he
never has any problems with his fish and he has a couple koi. I don't want
to lose my gf but if I do I'll only be out a couple bucks and I'll know
better for next year. I'm not sure what the temperature in the water is but
I don't have any ice yet, I took the thermometer out so it wouldn't break.

Dan
NW Indiana, Zone 5


Do you have any ice on the surface of your pond? If so, you may want to
add a de-icer or an air stone to keep a hole open so that you can have gas
exchange. Also, the last thing you want in northern Indiana in the winter
is to find your pond is frozen solid with 13 fishcicles in the middle of
it.