Welcome to the forum!

I think many ponds in the northern part of the midwest will experience winterkills this past (hopefully) winter. There's two things that you can do to prevent that. 1) Reduce the Biochemical Oxygen Demand on the pond 2) properly aerate the pond.

www.hoosierpondpros.com

[Linked Image from lh6.ggpht.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).