Observations have shown me that as long as there isn't a ton of wind, the water will chill from the bottom up, as well as the top inch or two will be chilled. That top layer gets heavy enough, it sinks in areas to the bottom, and you can see this take place at the surface when you have no wind with small waves out of nowhere. There will be a warmish layer between the two cold where the fish will hang out until the warm layer mixes out.

I would assume aeration would break up that warm layer, so that the entire column would be about the same temperature except for maybe the top couple of inches, slowing the bottom cooling but speeding up the warm layer cooling.

Personally I would turn off aeration during fast cold snaps, no need to speed up the chilling process and ruin the refuge in a warmish layer. The natural turnover of that top cold layer to the bottom will bring down plenty of dissolved O2. Of course these events are usually accompanied by high winds, so it probably makes no difference one way or the other, unless you are talking at night when everything has gone still.
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