Here is the deal. I have been working on my plan for my backyard. Living in town I cannot dig a real pond so I am thinking in the direction of a koi pond full of catfish and bluegill instead of koi.

My thought is to get a couple of he large (1000 gal) round plastic stock watering tanks from the local farm supply. Tarrace them so the top one flows into the bottom one and then a couple solar koi pond pumps pump the water back to a biological sand filter at the top. The tanks would also be incorporated into my rain collection system so that the irrigation to the garden passes through the ponds in affect giving the ponds a partial water change every time I watered my garden.

This Koi pond would serve several purposes for me.
1. Its 2000 gal of emergency water
2. Water garden and fish growing
3. It would be nice to have a pond of fish in my own back yard right off a back patio.

I have most of this worked out except for one fly in the ointment. How do you keep such a water feature from freezing in the winter? Ideally I want the fish to be able to live in this year round. and not have to empty and winterize it every fall.

The 1000 gal stock tanks are 29" deep and about 9ft dia I beleive. IT is a lot of water and is not likely to freeze completely if put in ground here. Our frost line is at 18" and the water will have a mild current flow to it as long as I can keep the pumps from freezing up.

The whole system would be relatively inexpensive to build except the winter strategies add money fast. An electric heater runs up the electric bill, A windmill and bubbler are expensive, I have thought of putting a fresnel lense shinning on a large aluminum heat sink setting in the water of each tank. I have thought of insulated covers, passive solar covers,....

How do folks deal with koi ponds in winter? Is there cost effective way to keep the water from freezing and busting a tank?

If your cool with running electricity for a pump then a heater such as is used for keeping livestock tanks from freezing should work. If you could berm up soil around the tanks and/or bank the outside of the tanks with straw or hay (really green hay might generate a bit of heat on it's own) then the tanks would be further insulated and the heater should not have to run as much.

Would love to see pics when you get this going.