A pre-filter is a second filter device that is placed before the main filter. It collects extra debris and adds some more bio-filtration, keeping this debris out of the main filter system. Though they can be beneficial for a larger pond that’s heavily stocked with koi or other fish, they aren’t necessary for smaller ponds with only a few fish. They also require a stronger pump since it’s going to need to push the water through two filter systems instead of one.