Mechanical filters remove solid waste and particles from your pond. They do not filter any toxins out of the water. Anything from nylon bags to leaf strainers can be used as a mechanical filter. The best filter systems usually have a mechanical filter near the beginning to take out all of the solid waste so it won't "gum up" the other components. Mechanical filters usually need to be cleaned out often so the solids they catch don't start to release toxins into the water.