FWIW, I run my summer diffuser at the deepest point in the pond (14' currently) and for the winter, I turn it off and turn on my diffuser that is suspended at 5' depth in 10' of water.

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[Linked Image from lh6.ggpht.com]
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3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).