Sometimes called a 'plant filter', a vegetable filter uses plants to process waste products and purify water. Often used in ponds where the pond water is pumped into a large, shallow bed of plants - as the water flows through the roots and soil, the plants remove waste products to be used as food. Water exiting the vegetable filter should have been stripped of common pollutants, and returned to the pond clean and filtered. A well-designed vegetable filter is an excellent low-cost and low maintenance method of filtration, with the main requirement being enough space for the plants.
Vegetable filters in aquariums are rare, although trickling water through lit containers of moss or other plant material has been used in experimental filters.