Common sense will tell you that if your pond is only 6' deep and it freezes 3' down nothing can survive.
This is an incorrect statement.
Common sense will tell you that fish that go without oxygen will die. Your logic is faulty. I'm aware of many of ponds and lakes that have seen 50% or more vertical freezing without winter kill. In fact, all of Nebraska's Western natural lakes have vertical freezes nearing or exceeding 50% of depth. While it's true that this increases the chance of winter kill, it by no means tells you that "nothing can survive".
