Next summer I would like to build a pond/pergola/fire pit. The pond I would like to be above/inground. I understand the frost line is 30''. I am having trouble understanding filters. I have been in aquariums my whole life. Even worked as a both a Service man and sales man on aquariums, but pond filters are throwing me off. Here is my idea. My goal is to build a pond that is 40'' in ground at the deepest, and 2' at the most shallow. The shape would be comparable to a surfline fish tank if you looked at it from above. The walls above are going to be made of cinderblock, coated with a real stone vaneer. I already decided on the firestonepond EDPM 45 mil liner. what i want to do is build a second "pond" alongside the long wall, and run water into it from the main pond, and use the second pond as a "wet-dry"filter. then just use a smaller pump to pump it up and form a waterfall. I was going to enclose the second pond, but still have some access points to fix problems later in life. Will this type of fiter work? or any other suggestions would be great. Thanks for your time. BTW the pond will be covered by the pergola, and and some vines so it wont get full sun all day.