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Filter media is anything placed in a filter that changes the quality of water flowing through it.  With the variety of media available, specific types can be chosen to create the optimum environment in your aquarium.      The three types of filter media are mechanical, biological, and chemical.  All three types are recommended, but an aquarium needs to have biological and mechanical filters at minimum.  Components of these media types can be incorporated in the same filter.  The three types of filtration utilize three different types of media to perform their functions.  This article will help you weave through the maze of terminology and differences between the media filters use. ABOUT MECHANICAL MEDIA      The components of mechanical media are inert - this means that the material this media is made of will do nothing to interfere with your water chemistry.  This media mechanically or physically strains solids from water passing through it, which is vital for the efficacy of your biological media.  It simply needs to be cleaned - replacement is only required when it can no longer release all the dirt that it holds.      Mechanical media is available in many different porosities, which control the size of the particulate that you can extract.  The larger the pores in the mechanical media, the larger the particulate matter must be in order for the filter to extract it.  This media, known as  coarse media, is easier to clean and reuse than finer media.  The pores in the finest mechanical media strain out particles as tiny as one micron, which is 1/90 the diameter of a human hair!  The finest media will clarify the tank, but will clog very quickly, and may not be reusable.      Mechanical filtration removes unsightly particles from the tank.  This may include fish excrement,sludge, uneaten food, or dust.      Aquarium water is passed through a mechanical filter, and the particles are strained out.  To prevent build-up, the filter media must be cleaned regularly. ABOUT BIOLOGICAL MEDIA      Biological media is anything inert that provides housing for beneficial bacteria that break down dissolved solids to a less toxic form.  This is a media that should not be replaced unless it has become too clogged to function.      To understand this media, first we must understand how bacteria thrive:                1.  environment above 55 F                2.  need ammonia or nitrite as food source                3.  need oxygen for life      Bacteria extract oxygen from water moving by them.  When particulate matter gets into the media, decreasing water flow, it "starves" that area of oxygen and food, causing bacteria to die in that area.  This is why mechanical media is suggested before bio media.      When the bio load increases, the abundance of ammonia and nitrite causes the bacteria to reproduce.  Bacteria that consume ammonia reproduce quickly, until the ammonia level drops to zero.  Sometimes they over-produce and cause a "bacteria bloom" that looks similar to smoke in the aquarium.  Once the correct amount of bacteria have grown to overcome the ammonia spike, the excess bacteria starve and die, and the "smoke" disappears.      In an established tank, a bacteria bloom is less likely to happen and any ammonia spikes may be gone in 24 hrs-often before you even have time to test for it.  In new tanks the time the ammonia takes to be converted by bacteria depends on the amount of fish load and bacteria you start with.      A good source of bacteria is a scoop of surface gravel from an established healthy aquarium or some bio media from the filter, as long as the aquarium hasn't been treated with medications or other chemicals.  Another choice is a bacteria additive.      Once the bacteria have become established in the biological media, they are difficult to destroy, except by over-cleaning, using chlorinated water, or using certain medications.      As the ammonia-eating bacteria consume ammonia, they give off nitrite, and then in come the nitrite-eating bacteria.  These bacteria are slower to reproduce, preferring to wait until ammonia levels are zero.      A variety of media is available for bacteria housing.  Small biological media are extremely porous and boast incredible surface areas.  They can accommodate a lot of bacteria in a small space.  They also can get clogged easily and lose their effectiveness quickly if inadequate mechanical filtration is used.  Smaller media last 2-4 years, and are excellent option if you have a small tank.  Plastic media do not have the extensive surface area, but they are unlikely to clog and never need replacing. ABOUT CHEMICAL MEDIA      Chemical media is not used as often as bio;logical or mechanical media, but can be effective for an assortment of filtering purposes.  This media encompasses a variety of materials that can remove one impurity or all, and everything in between.      The chemical method of filtration removes dissolved particulates from the aquarium via activated carbons, resins, and other adsorbents.  Chemical filtration media causes the dissolved matter to adhere to them.  The two most popular forms of chemical media are activated carbon and resins.  Protein foam skimming or oxidation with ozone are two other forms of chemical filtration. ACTIVATED CARBON      Carbon is filled with microscopic pores that cause certain organic or inorganic materials to stick to them.  New carbon works more efficiently than older carbon.  When all of the pores are filled, the carbon is no longer effective and the filter begins to act as a biological filter.      Carbon removes many harmful elements from your aquarium, such as: copper, chlorine, dissolved proteins, and carbohydrates.  It also removes sulfa drugs and antibiotics, so it should be removed when the aquarium is being treated, then replaced when treatment is completed to remove any leftover medications.  Activated carbon can make your aquarium look so clear by removing discolorants that you'll question whether water is still in it. ION EXCHANGE RESINS      These filter media are less utilizes than carbon, but are becoming more common.  These work by attracting a specific molecule to adhere to them.  Some attract ammonia or nitrate, and some remove dissolved organics.  Ion exchange resins are also utilized in some carbon mixtures like Chemi-Pure and BioChem Zorb.  The the resins often strengthen the filtering ability of the carbon, as well as help biological activity by removing pollutants before they enter the nitrogen cycle.      Used properly, chemical filtration can be one of the most useful tools of the aquarist.