| Kimberly I live in Northern Ontario and built my first pond 4 years ago. I started with 4 3" Koi and about 20 2" comets. As the winters here are probably as harsh as they are where you are and my pond is only 27" deep I had to overwinter them in the house. The first winter I put them in 1 33 gallon and 2 20 gallon aquariums. They did well (frequent water changes were needed), however by the next winter they had grown so much that aquariums were pretty well out of the question. So, for the last two winters they have spent the winter in my basement in a 300 gallon pond that I contructed out a a piece 0f 10' x 10' liner and a wooden frame. The pond was 6'x6' and 18" deep. I used the pump and filter from my outdoor pond. The first year in the indoor pond they thrived, however by the second year they had grown so much that even with daily partial water changes they were starting to show signs of stress, but they did make it through the winter. My four koi have grown to about 14" each and my goldfish are about 6 to 8" making it impractical to overwinter in the house.
I spent this summer building a 5,000 gallon pond with a deep end that is 40" deep and 8' wide. Over this deep end I have built a shed that is 10' x 10'. I have a 4'x4' opening in the floor of the shed. I am hoping that with the heat from the sun through the windows and with a couple of floating in pond heaters I will be able to keep a hole open in the ice. This may seem like an extreme and expensive proposition, but I have become very attached to my Koi and besides I had a need for a shed anyways. With that many Koi, at some point in time it is going to become nearly impossible for you to overwinter your friends in the house. Good Luck | 
