Almost no way that environmental-based temp changes (i.e. the water in a big matching the outside air, with only a few degrees of difference) is going to drop the temps enough to do serious harm. It's the lack of water movement in a reasonably crowded system that sends things downhill, even sometimes in just a few hours. Someone correct me if I'm wrong, but lack of any real gas exchange probably doesn't do wonders for the ph levels either.
But even a single pump running is more than enough for most tanks to provide the necessary oxygenation, especially if it ripples the water surface at all.
blueeagle: yeah, most UPSes in a reasonable price range aren't designed to run a long time in an outage under a heavy demand load. Still, if you only plug one or two pumps into it then it will last pretty long (hours maybe for a good one). You can also get battery powered air pumps, the more expensive of which will switch on automatically, or switch from outlet to battery, when they lose power.
As long as temperatures will stay in the normal species range even without a heater, you really don't need too much in the way of circulation to keep a tank alive through an outage.
