The biomedia within the filter provides surface area for bacteria to adhere to. The more surface area, the greater the number of potential bacteria. The water flowing through the filter brings oxygen and food (nitrogen) to these bacteria. Surface area, nitrogen load, temperature and water chemistry will ultimately control bacteria population levels

Simply put, the bacteria convert the toxins released by the fish to far less toxic plant foods. Your filter should also be effective in removing particulate from the water. This is achieved by allowing heavy detritus to settle in the first chamber. A vortex is utilized here to help facilitate the sinking of solids to the bottom of the tank for easy flushing

Read more about this in Design Concepts below

Store bought filters
These do offer some advantages. They have manufacturer support, usually a better finished look and are easier to setup. But consider proven full size units only. The less expensive store bought filters ie. in pond box and small pressure filters - are excessively overpriced, fundamentally flawed and maintenance nightmares. Value is what you are after

Poorly constructed filters use foam as the initial filter media. Unfortunately the foam is quickly saturated by debris. This means the flow rate (ie return to your waterfall) slows down and you will need to clean the filter very frequently. In a properly designed 3 chamber filter, the foam goes in the last chamber so that it only has to trap small debris - larger particulate has already been settled out. Therefore much less frequent cleaning is required and water recirculation remains at peak rates

Some companies claim that natural filtration with rocks on the bottom of the pond and waterplants are all you and your fish will require for a healthy balanced eco system. The truth is however, You can't rely on a natural process in a unnatural setting. A backyard pond is not a natural thing. It's a small closed system with a high bio load and our goal is that everything survives. No one I know is willing to keep 1 fish per 10,000 gallons+ so natural approaches will fall short. Sure a microbial web will develop in any pond but it is not extensive enough to cope with the abnormally high fish load. Rudimentary science debunks this marketing myth espoused by companies who just want to sell. As with keeping all animals, hygiene is important. I don't know anyone who keeps a rabbit or guinea pig and never cleans the cage. Allowing the natural breakdown of the high excrement load in a fish pond is asking for disease issues

A quick tip: if you are planning on purchasing a filter. If it easily fits in the backseat of your car, it's really not designed to filter a pond

Best way to supply the Filter A bottom drain is really the key element required in an effective filter system. If you are pumping the water to your filter from the pond you will not be able to remove large debris Additionally, a pump in the pond will get clogged over time decreasing the turn over rate. Debris that manages to go through the pump will be broken down into smaller particulate making it harder to settle Bottom drains are the best way to keep the bottom of the pond clean and help maximize water movement throughout the entire pond

The tank pictured left is 300 gallons and has a conical shaped bottom to allow proper flushing of accumulated debris. This is a classic vortex and well worth the investment

Gravity Fed Filter Systems
The Foundation
I consider gravity fed filter systems to be fundamentally sound and proven. Every pond should have a gravity fed system in place and the addition of other filter types should be strongly considered

Recommend Filter Types
Bead filters have there merit but there are caveats. They still require extensive settling and pre-filtering before water is pumped through them. They are excellent for small particulate removal and biofiltration - a nice complement to a gravity system. As they clog the flow rate is reduced therefore Bead Filters need to be monitored
and cleaned regularly

Trickle towers, used previously for marine tanks more than ponds, towers are in vogue now and do provide a super saturated biobed with a small footprint. They are great at removing nitrites. Because the media is not submersed, the bacteria will not survive long if the water stops flowing through the tower

Rotating media filters offer very good nitrogen conversion and are inexpensive and easy to build. We install these on many of the higher end ponds we construct. Its hard to find a negative with these types of filters