Any materials you place in your canister filter to change the quality of the water are considered filter media. The conditions required in your specific aquarium will dictate which types of filter media you will need and also which order to pack them in. Yet the general guideline is based on removing ever-decreasing particle sizes while still maintaining the health of the beneficial bacteria.
Keep the basics in mind and experiment with combining different media types, but maintaining this overall order will achieve the best filtration for your aquarium.
Mechanical filter media should be packed in first so that it receives the flow of raw water. All water should come in contact with this mesh part of the filter before it reaches any biological or chemical filter media. Mechanical media is intended to remove larger particles from the water, thus eliminating clogging in other areas of the filter. Placing it first in line allows much of the debris to be captured before the water continues on to the other filter media.
Types of mechanical filtration media include filter wool or floss, bonded pads, or sponges. While some of these can also serve a dual purpose as biological media, it is important to remember that any mechanical filter media must be removed and replaced frequently, so some bacterial colonies will be lost.
Optional Dual-Purpose Media
Some owners prefer to use sponges and pads material as both mechanical and biological media. When dual-purpose media is used, you need several layers, preferably of differing degrees of coarseness. The coarsest material is placed in first.
Because the sponges are serving as both mechanical and biological media, each layer should be cleaned at different times. By staggering the cleaning schedule, they will have time to repopulate the cleaned zones. This way, there is less overall disruption to the bacterial colonies.
Biological media (bio-media) is usually placed after the mechanical media but before any chemical media. The biological media must not become clogged, as it serves as the right substrate for colonies of beneficial bacteria. If clogged, the oxygen flow will be impeded, putting the bacterial colonies at risk of die-off.
Biological filtration media are like condominiums for bacteria. Types include ceramic rings, sponges, pads, bio balls and bio stars, sand, and gravel. Keep in mind that some of these materials can also be used as mechanical media. These porous objects have high surface areas while still allowing the passage of water and light to flow through them; they make excellent homes for bacteria.
Different species of bacteria grow into healthy colonies all over the surface of the bio-media. There, they help to break down (eat) various toxins in aquarium water. Ammonia, nitrate, and nitrite loving bacteria remove a large amount of these toxins, just as they would in a natural ecosystem. Bacterial colonies also consume dissolved organic solids such as proteins, sugars, and other carbon-containing compounds.
Many aquarists use chemical media to remove toxins and alter water chemistry. When used properly, chemical media is placed in the final portion of the filter. If chemical media is placed first, it will become clogged and need to be replaced long before its chemical filtering properties are exhausted.
More importantly, placing chemical media before the biological media means that what usually feeds the bacterial colonies will be removed prematurely, before reaching them. As a result, bacteria will be less likely to thrive in the biological filter media. Placing chemical filtration media at the last stage of the filter ensures the bacterial colonies are not affected.