Thanks AnitaPond,
It looks like Tacoma is in zone 7. According to the chart, we get temps down to 5+/10+ F. I have lived here most of my life, and I don't remember ever getting down to 5 degrees F. It's hard to say who came up with these charts. That's why they confuse me, I'm sure some chart maker is misinformed. Thanks for the help anyway.
I have been using a 150 watt submersible aquarium heater (s.a.h.) in my ponds over the last several winters. I started to a s.a.h. in my first above ground tub type fish pond, because I knew that it would really freeze up without some assistance of heat. Now my pond is a 365 gallon inground pond with a 27 gallon above ground boi-filter. So in addition to the 150 watt s.a.h., I'm adding a 300 watt s.a.h. to the boi-filter when the temps start to get near freezing. I always set the heater temps to keep my pond water temp just above freezing. I like the temps to be in the low 40 degree F range.