Our current koi pond is 14 years old. it is situated in a Japanese garden of our own design and construction. I'm currently retro designing and out of pond pump as my 4th sump pump failed. The pond is a rubber liner type with marginal shelf. It is just shy of four feet deep. We incorporated a two stage waterfall. The first is up inside a juniper tam in a grotto format. It falls 3 feet then streams down 6 feet and the secondary fall enters the pond with a 8" drop. The bio-filter is behind the main fall.