Vegetable Filters

Vegetable filters are becoming increasingly popular as they provide a means of reducing the nitrates in the pond and thus reducing the problem of blanket weed. They are also quite an attractive feature in themselves and a place to display marginal plants away from the fish. Their vigorous feeding habits may mean that the plants may get eaten, uprooted or even fail to get established, if they are planted in the shallows of the pond itself.
When deciding to install a vegetable filter remember that it will need to be large enough to deal with the nitrates that the pond filter is producing, 25% of the ponds surface is recommended.They need to be run as an addition to your bio filter which will remove solid matter from the pond, the veggie filter finishes the process by the removal of the nitrates.
The success of the vegetable filter depends upon the plants used. Although all plants will remove nitrates from the water, as they use them for nutrition, some are more efficient than others. The top recommendations are;

Norfolk Reed - Phragmites australis - as this is a fast growing plant it uses up nitrates and phosphate nutrients very quickly, making it an ideal plant to have. During daylight hours it also produces plenty of oxygen so has a double advantage.

Sweet Galingale - Cyperus longus - a quick spreading plant, can be controlled by cutting down to the roots.

Soft Rush - Juncus effusus - another fast growing plant, can be invasive so needs to be kept under control.

Bullrush - Scirpus lacustris

The Flowering Rush - Butomus umbellatus - despite its name, the flowering rush is not closely related to true rushes but is, in fact, a unique flower with a genus and family (the Butomaceae) all to itself.

Some other plants that do well are; Water Cress, Rorippa nasturtium-aquaticum, although it can become a problem since it is capable of growing rapidly and "taking over" and once installed can be difficult to eradicate, Bittercress Cardamine praetense and Fools Water Cress, Apium nodiflorum, which thrives in alkaline water.

The plants described above are normally classed as marginals. These plants grow in marshy areas and like their roots to be wet at all times, they range from those that grow in wet ground to those that will grow in shallow water. They grow naturally on or in the edges of lakes, ponds and slow moving rivers.

Installing a vegetable filter can be as simple or as difficult as you make it, depending on the way you want it to look and whether it makes use of what you have already or is built specially for the purpose.
If you already have a pond that has a stream or waterfall with header ponds you can simply place aquatic containers that have been planted with an assortment of the plants mentioned in strategic places. Keep an eye on them and make sure that the roots do not spread under any rockwork.
Alternativley you can have a purpose built vegetable filter that is decorative in it's own right, you will need to pump the water from the bio filter through the plants in the veggie filter and then back to the pond.

A veggie filter can be made from a large container, for example a header tank, or a series of these for a larger pond, as long as they can be connected together by inlet and and outlet pipes. Alternatively you may decide to build an extra permanent 'pond' to act as the veggie filter.

Whichever you decide they work in the same way, the water is pumped from the bio filter, to 'trickle' down through the plants, which are planted in a gravel bed and then back to the main pond.
Once set up the system will begin filtering straight away, but allow at least a month, in the growing season, for it to reach peak profficiency and to give the micro-flora chance to develop and get established. During the winter months when there is little plant growth there should still be a certain amount of filtration happening from the bacteria in the gravel. In very cold weather it may be necessary to switch it off but any algae that develop in the pond will soon be eradicated when the filter is switched on again.
Plants should be cut back at the end of the season, plants such as cress may need to be cut back on a more regular basis.
Dig the plants out, divide and replant in fresh gravel every 4 years.