Actually, I am building backyard ponds for a living, and you don't need to go as deep as one might think. We build ponds 2 feet deep and some go as little as 18" and they keep fish over winter. Seeing that we are from the great white north anywhere below here should be the same! Reason being is that water is an insulator, and so is the soil. Water has takes a lot more to freeze especially after it is under the ice already. But nice looking pond tho. I believe it may be a little under filtered for the size that it is. With a pond you want to turn the water at least 2wice an hour, Yours at best is only doing 1 and a half.