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The plant filter is based upon the plant's natural ability to assymilate certain organic compounds necessary for their growth. Most of these compounds can be found in aquarium water as the by-product of the fish's metabolism. Among these are nitrogen and phosphorous based compounds which are undesireable in the aquarium. While nitrifying bacteria can convert nitrogenous waste to less harmful compounds, plants can literally remove the waste by converting it to new plant growth. Plants also use phosphates in growth; nitrifying bacteria don't remove phosphates. By keeping a sufficient amount of plants in the aquarium, it is possible to keep these compound levels very low. This not only helps keep the fish healthy, it also retards the growth of algae. It is possible to use a more aggressive approach, however, than simply using aquarium plants. A plant filter acts as a wetland or bog, where emergent plants use nutrients from the water to grow, purifying it in the process. A properly functioning plant filter can be much more effective than the typical wet-dry or other bio-filters in use for aquarium maintainance. As a bonus, it can also be set up in a manner that is esthetically pleasing; in other words, it can look nice as well.

To build a plant filter (there are none on the market yet, to my knowledge) you need to know a little bit about the engineering behind a typical overflow-type wet-dry filter. In essence, you need to rig an overflow box to a trough in which the plants will grow. The water will flow through the trough to a reservoire where it will be pumped back into the aquarium (see figure).

A light source must be provided for the plants to grow. This may be a window, an electric light designed for growing plants, or a combination of both. The plants used must be able to grow with their roots submerged. Bog plants or pond periferals work well, as do many house plants. Round, smooth rocks fill the trough and give the roots something to anchor to yet leave enough space between for water to pass through. Water flow need not be rapid, but should be enough to prevent dead spots from forming where the water will become anaerobic and rot the roots of the plants. When choosing plants, try to select fast growing types to maximize their waste removing properties. Good plants includes;
Ficus species; weeping fig, climbing fig, etc.
Pathos vine or philodendron vines
Brazilian swordplant
Bamboo and emergent grasses
Some iris species
Some ferns

There are others, too, so try experimenting until you find what you like. A plant filter does have some drawbacks. Among these are increased evaporation of the aquarium water and the use of trace elements, especially iron. If you are using a plant filter or any other plants in your aquarium, you should make regular additions of a nitrate/phosphate free aquarium plant supplement, or the equivalent. If the new plant growth appears pale, then there is insufficient iron present.

A resourceful and creative person could create a window sill garden that would double as an aquarium filter. Taken even further, one could create a terrace of steps and falls, with each level planted beautifully with verdant foliage. Using items found at most garden and hardware stores, this sort of project need not be costly or difficult to build to be effective and attractive.

by Kent Turner

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