Dwight up in Minnesota has a 3 stage geo system like you want to use (closed and in the pond). I'll bet if you send him a PM he'd tell you about it. He said that even in sub zero temps the electric 3rd stage hasn't kicked on yet.

www.hoosierpondpros.com


http://www.pondboss.com/subscribe.asp?c=4
3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).