In a tragic turn of events, my winter season is over. The circuit breaker for the greenhouse tripped and it went through the night with no heat.
The outside temps went down to 21F (-6.1C) last night while the inside temps went to 27F (-2.8C). Everything got frosted except a few lettuce plants. Tank water is at 38F (3.3C)...goldfish are conserving energy by sitting at the bottom of the tank. They'll be perfectly fine this way until spring.