Pond filters sit either outside or inside a pond. Some people prefer to use both types. A submersible pump circulates water from the pond through the filter and back into the pond. Often the submersible pump, which goes in a pond, is contained in a filter. A drum filter usually sits outside a pond.
The most important aspect of any pond filter is the filter material, or media, used. Many people experiment with different kinds of media until they find one that works well for their specific pond filter. Types of media that have worked well include lava rock, plastic pot scrubbers, non-fiberglass furnace filters, and foam. Generally, a combination of these will keep your pond water clear and provide a bio-filter for good bacteria to grow. Good bacteria help fight disease and keep fish and plants healthy.
Plastic drums work well for pond filters. Plastic is usually non-toxic and will not rust. Plastic drums (55 gallons) are have either a closed or an open top. For a pond filter, use an open top so you can place the media inside the drum. An open top will also make it easier to install valves and tubing.
Make a hole for a valve toward the top of your drum. Attach the hose from a submersible pump to the valve. Use a sealant to fill in open areas around the valve. Install a valve toward the bottom of your drum and a hose that will go back into your pond.
Essentially, the water from your pond goes through the top valve, down through the filter material, then out the bottom valve and hose back into the pond. The bottom hose can go directly into the pond or be connected to a fountain or waterfall.
A good media to use in a large drum is pot scrubbers. Fill your drum three-quarters full with scrubbers, and covered the scrubbers with foam batting (used in quilting).
If the media in your submersible pump stays dirty most of the time and is not doing a good job of keeping the water clear, the filter may not be big enough for the pump. Add another drum by connecting a hose between the two drums. Two filters may also be needed for larger ponds (1,500 gallons and over).
If the filter media stays clean and does not look used, and the water in the pond is still not clear, consider getting a larger submersible pump.