Based on what iv´e read, one of the most effective "build your own filters" is the following:
You buy a circular or oval shape farm animal feeder from rubbermaid or any other brand, then you create two holes: On the bottom part one for water drainage, and near the top part, one so the water goes back to into pond. The water coming from the pump, goes through a PVC that goes through the middle of the rubbermaid downwards, and at the bottom of the PVC you have a T shape, with to outputs of water, both outputs in opposite direction to each other, so you can create a Vortex effect of water going to the top (antigravity). You also have to use eggcrate in the near bottom part of the Rubbermaid, but not all the way in the bottom. The eggcrate is usefull to put the filter material, and to make enough space for the dirt to set up in the bottom, for later drainage.
An excelent filter material is square shaped floor scrubs, these provide excelent mechanical and biological filtration at the same time, they are cheap to obtain. One you have all this, don´t cover the filter, since the beneficial bacteria from the floor scrubbs also use the sun and compete with the green algea.
