Question
I cheat with my koi.

My pond is only 100 gallons, but very well filtered and aerated (yeah Ebay)....

But because of the size, I can't overwinter them. I have built a fish cage, if you will, and every late fall I sink them into my parent's 3/4 acre pond.  Their pond is wonderful for them, they are below the ice, it is aerated by windmill, and they grow like weeds.  They have spent 4 winters this way and seem none the worse for wear.

I figure I must have the heartiest koi ever (and a couple of shabunkin goldfish too)

-Beth

Answer
Very interesting.... I used to purchase Mark Weiss Products for our Marine Fish Tanks and bought a few goldfish food products for a friend's Koi pond several years ago.  Wish I had one of those.  Very hard to dig in our yard, lots of very solid, compacted clay.  If I could, I would have a setup just like yours.

I bought a Rototiller way back but it still had gas in it last fall and I didn't get around to using it all up and putting it in storage, so who knows how I will ever get a 4' pond into the ground.  

But one can dream.