This is my 120 gallon turtle pond
As you can see, it is one of those pre-made pond-liners in a box make of landscape
timbers with sand to stabilize it.
And Technically, you can even call it planted

(below is what I mean by "planted")
The filtration is a submersible pump with a prefilter that serves as mechanical and
bilogical. The pump is plumbed with an output around the back of the pond and then
flowing out on the side of it for gas exchange.
It is very stable and requires the pre-filter to be cleaned every 2 years and a full
water change when the pollen from all of the live oak trees starts to drop. Oh,
and every other day of tossing a capful of pellets into the water for the turtles.