On Thu, 09 Feb 2012 10:33:47 +0000, F wrote:

My only concern is that it would tend to turn the water over and lose
the 4ºC 'warm' layer at the bottom when the top is freezing. Is this an
issue at all?

Could be avoided by having the airstone only an inch or so below the
surface? It's the movement that stops the freezing, hence people
keeping waterfalls or fountains running.

--
Cheers
Dave.