This is pictured at the top left of this page with the two schematics below it.  This filter is an 18 gal. Rubbermaid tub fitted into a pressure treated wood I made.  I used Styrofoam blocks and spray foam to stiffen the sides between the wood and the tub.  The filter has two sections to it, one has a dense filter mat to filter fine particles and the other is full of plastic BoiBalls.  The water runs through a venturi which oxygenates it then it goes through a bag of barley straw before passing through the first section.  The water leaves the filter via a flow regulator I made by loosely securing a piece of scrap plastic around a perforated PVC pipe with several nylon ties.  Depending on how I turn the plastic over the perforated pipe I can maintain a constant depth in the filter which slows the water enough to allow the bacteria to do it's thing.  When the water exits this filter it enters another bio-filter I made out of a whisky barrel liner.  You can just see the tip of the PVC pipe and pump under my little foot bridge.

This filter is pictured at the top right of this page with the two schematics below it. The original intent was simply to increase the flow rate of a near by falls by adding an additional pump to the stream below (I have two ponds dumping into this stream with one not being an original part of my system..)  Anyhow  I basically built this filter with some old bricks, egg crate light diffusers, and a rubber roof vent boot.  The boot seals the pump side of the filter and a PVC pipe from the pump fits snuggly into the boot creating a solid seal.  The whole thing , aside from the pump and tip of the boot, are buried in several inches of random sized river stone/pebbles.  I can even flip the pump around and back flush the whole filter if I want to -which I have yet to need to do in over 5 years...