Place the airstone toward the prevailing wind side of the pond. Wind blows the warmer water lifted by the bubbles, so that the water will be open farther away from the airstone on the downwind side. If your wind direction doesn't shift much, this could give you a noticeable increase in open water size.

"Live like you'll die tomorrow, but manage your grass like you'll live forever."
-S. M. Stirling
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