[quote=Capn Courage]Has anyone tried using rigid insulation floating on the pond to keep the
water temp up a few degrees. I normally run a small pump to keep a hole
open
for gas exchange but last winter it was so cold here in southern Ontario
that it froze over anyway. My pond is 1800 gallons and 3 foot deep with 50
to 60 fish of all sizes up too 18 inch's. Any suggestions would be greatly
appreciated.

Rigid material? Why? it would reduce the sun warming the water and add to sealing the water surface, contributing to poorer water quality.

Now if you put a sturdy frame (sturdy enough to cope with a heavy snow) a foot or two over a pond with a strong grade of clear plastic sheet (6 mil) that would collect warmth from the sun and reduce ground cooling after dark.

That would significantly improve a ponds 'climate' in harsh freezing Winters. Take care a 'cold frame' does not bake and broil the pond, when Spring sun gets to strengthen up...

Regards, andy
http://www.members.aol.com/abdavisnc/swglist.html