Thanks for all the replies.

Just as an FYI, I have chinese grass carp in my outdoor pond and it freezes solid every winter and they survive. It is spring fed, however, so maybe that provides some oxygen?.. As far as my fish tank, I plan to keep it circulating anyway and it is in a greenhouse so I doubt it will ever freeze, except maybe a skin of ice some very cold nights.

I think I may try to get at the nitrates with some floating lettuce. This link I found fascinating.

http://www.youtube.com/watch?v=FHBhyqowSEc

I was thinking of doing something like floating a large piece of styrofoam with holes in it for the lettuce. This will keep the algae down somewhat by blocking light and by sucking up nitrates..

The duckweed idea I like too but I worry that when I feed the fish, the food pellets will get stuck on the duckweed and not drop in for the fish to eat.

I still need to update my pictures to show my biofilter, blue gill and greenhouse!

Thanks,
Wes