04-20-2010, 07:09 PM
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Different filter media perform different tasks in filtration. Carbon is just one type of media; others include the pad or wool (floss) that all filters (except plain sponge) have; sponge is another; and biological rock like lave rock or biomax; ceramic disks; etc.
There are three types of filtration, mechanical, biological and chemical. Mechanical involves the removal of particulate matter from the water as it passes through the filter media. Biological is the colonizing of the media by nitrification bacteria that consume ammonia and nitrite. Chemical involves altering the water chemistry and carbon does this by removing dissolved (as opposed to particulate) substances from the water.
All aquaria, planted or not, usually benefit from mechanical filtration. All you need for this is a flow of water through media like a sponge, pad, filter wool, to remove the minute particles; larger particles will be removed when the water passes through the ceramic disks or rock materials, depending upon the type of filter. This mechanical filtration keeps the water clear--which is not the same as clean, a different thing.
Biological filtration occurs on all filter media as it also occurs on every hard surface covered by water in the aquarium--plant leaves, gravel/sand grains, rocks, wood, tank walls.
Chemical filtration is usually employed more in non-planted tanks. Plants do a tremendous job of filtration, and there is no need to interfere with that, as nature does it for us. And as Johnny correctly pointed out, chemical filtration sometimes removes essential nutrients that the plants need. So in planted tanks, or tanks with plants, chemical filtration is normally not used. It is useful after medication to remove the chemicals, which is why medications always instruct you to remove carbon from the filter before using.
So to answer your question, many aquarists use only mechanical filtration, especially if there are plants in the tank; and the more and larger the fish, the more mechanical filtration there should be. Biological filtration will occur on its own to varying degrees, and in non-plant tanks should be encouraged. Chemical filtration depends upon the fish and other factors, and continual use of carbon may or may not be warranted. Instead of carbon, some put more filter pads/wool in the space; some use plain gravel or rocks (this encourages biological filtration by providing more surfaces for the bacteria to colonize).
Some of the above may be old news, but I wanted to ensure the process was understood in its entirety since the type of filtration you need depends upon several other factors.