I'm thinking you're going to want to bring them indoors.

I don't know what the frost line is in Chicago, but in Minnesota it's
about 4 feet. My pond is about 1700 gallons and it's deepest point is
32 inches.It's built into a hill so half of it is exposed. I get
almost a foot of ice before the snow starts providing some insulation.
I use a 1200 watt trough heater to keep a one-foot tunnel open for the
air exchange. I spend about $100 a year to power it to keep my $0.25
feeders alive. I've had the current batch for 5 years and they're
about 8 inches long.

The heater gets deployed in late October and keeps chugging away until
late April or early May. One year the heater sank, the hole froze
over and everybody died. I tried bringing them inside the next year,
but then my fish season is only June to September and that doesn't
make sense. I suspect they are happier being comatose half the year
rather than being cooped up in an aquarium. After all, the fishing is
pretty good in Minnesota.

With 20 inches and 150 gallons you don't have much room to work with.
I don't think a heater will help. Time to plan for a bigger pond.

dss