How deep do i need my pond to be?

There are a lot of things to consider when digging a pond. In your case tempature is the main factor. I would make it no less the 3 and a half feet. The fish need to be able to swim to the bottom to keep warmer than the frozen solid top. If Margins means layers or tears in the pond, I would say two. I have three in mine. Plants provide a lot of things for fish and it is better to have a little than none at all (shelter, food, and hidding places for the young). Good luck.