We don't have waterfront property yet so we make due with a little pond.
DIY Pond Filter. Build a pond or aquarium fillter from a bucket and and a bunch of plumbing parts. The total price was less then $40. (not including the power head pump. I got that from ebay.) Took about 1 and a half hours to build. Not including the 2 hours spent looking for parts at the home building plumbing department.