Why are you convinced the goldfish would die if left where they were? As long as the tank water did not freeze (doubt that since the building should not get that cold) and the temperature drop was gradual, then they should be fine. At lower temperatures, the need for food would be reduced. No food means less waste.
When I kept goldfish in a pond I would typically hold off feeding from mid-October to March/April (water temps under 50 consistently). Just something to consider in the future.
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