I run my winter diffuser at 1/3 the deepest pond depth.

I have seen a pond with winter water temp at 37�F for a month or so, but not colder than that.

www.hoosierpondpros.com

[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).