First off, my pond is about 500 gallons, 28" deep at the deepest part
and I am in Zone 5 (Chicago).

I lost all my fish my first winter even with a bubler. A bubbler would
not keep a whole open. I even had a product that was supposed to not
use a lot of energy and keep a whole open (I forget the name), and that
didn't work here either.

The 2nd winter, I used a large Rubbermaid container (maybe 50
gallons), my outdoor filter, a bubler and brought the fish inside. It
was okay, but 1) had to feed them, 2) had to change/add water
frequently and 3) created some odor and a lot of humidity in the house.


This is the 3rd winter, and I decided the fish are too big to bring in.
I bought a pool cover (basically heavy bubblewrap) and covered all my
pond except a strip about 2" by 20" for air to exchange. This helps
keep the heat in and supposedly reflects heat in during the day. I
also have a bubbler and 2 300W heaters. I have only used 1 of the
heaters, except when it got very cold. For a few days, the pond
started to freeze over completely, so I plugged in the other heater for
a few days. I also added a second bubbler as on day at the end of
January I saw 2 Koi up by the opening, even though it was only about 35
degrees out. I thought maybe they needed more air. I placed the
bubblers and heaters near the deepest part of the pond where the Koi
will congregate. At last look, all is Okay.

Basically, my advice is to use a multi-tiered approach if you are in a
very cold area and can't bring in the fish. Use a cover (on the
surface or build a frame above the pond), use a bubbler for air and to
help prevent freezing, and use a heater for the coldest times.