Filters are not the sexiest part of your fish tank; most of the time, they're the part you'd just like to forget about. But we all know that choosing the right filter, and maintaining it properly, is the key to the health of your aquarium. So let's look at the three types of filtration systems that the average fresh water fish tank can use: First, there's biological filtration - that's what we call all the helpful little bacteria which break down ammonia and nitrite into less toxic nitrate. These bacteria need lots of oxygen, and a surface to cling to, whether that's plants, filter media, gravel, or live rock and sand (that is, rock that's inhabited by microorganisms). Remember that antibiotics will kill good bacteria as well as bad, so move a sick fish to a separate hospital tank before you treat it! Biological filtration alone is enough only if you have a very small number of fish in a large water volume - if you want a lively tank, you're going to need one or both of the other types of filter. The second type is chemical filtration, which uses chemical reactions to remove waste substances from the water.
Activated carbon, Algone (plant fibers), and zeolite clay (the type used in cat litter) can be used, and each type has their advantages and disadvantages. Carbon can sometimes leach phosphates into your tank, which promotes algae growth - look for brands that are "low ash" or "acid washed" to avoid this. Zeolite is good at removing ammonia, but can interfere with the establishment of a good nitrogen cycle, so be careful about using it in a new tank. The third type is mechanical filtration - that's just your basic porous filter that catches solid particles before they can go back in the water.
This type doesn't do anything about liquid or gaseous chemical wastes. These work best if the filters in them are replaced or thoroughly rinsed every two to four weeks, otherwise they get clogged, or the wastes will decay in the filter until they're small enough to get washed back into the tank. Remember that a mechanical filter won't affect wastes that have settled onto your gravel; you'll need a siphon or vacuum to get rid of those. Personally, my favorite type of mechanical filter is bonded floss - it doesn't get clogged as easy as the other types, and you can change part of it at a time to preserve your beneficial bacteria colonies. You can also get loose floss, sponge, or paper cartridge filters. You just want to pick one that traps enough solids to keep the water clear, without getting plugged too often.
Together with Iszuddin Ismail, Mic Hudson is sharing his wealth of knowledge on saltwater and freshwater tropical fish tanks. You can start with a free ebook on decorating your fish tanks. Download Fish Tank Aquascape at http://www.PetFishGuide.info