Peter May says...

Vegetable filters or simple reed bed systems are an ideal supplement to any pond that has a high pollution potential or a large population of fish. Although most biological filters can process the waste from fish into nitrates, it is getting rid of those nitrates that most systems have difficulties with. This is easily done by the micro-flora around the roots of higher plants. Just planting up a stream or the header pools in a waterfall system can be beneficial, or even planting up the surface of your existing biological filter is better than nothing. A purpose-planted container where the polluted water rises up through a porous planting medium, or gravel planted with fast-growing water plants is ideal. The best plant for this job is the cheapest, and it also grows quickly: the common or Norfolk reed (Phragmites australis). You will need a submersible pump capable of handling solids and pumping the complete volume of the pond water every two hours, 24/7 to the top of the filter bed. If you are incorporating this with an existing biological filter you already have the most expensive equipment. All you need to do is run the filtered water through a gravel bed planted with common reed. Plant it no more than 20cm apart straight into the gravel or leave it in aquatic baskets for easy maintenance. The beds need be no deeper than 25cm, with a total surface area of no more than one third of the pond’s surface area.