In any type of filter system you always want the progression to be;
First should be mechanical filtration;
sponges, filter floss, filter socks, protein skimmers and strainers are all forms/types of mechanical filtration in that they remove particulate matter from the water column and segregate it in a specific place.
In canister filters it is often best to use 2-3 sponges of increasing density for the best mechanical filtration.
Second in the progression should be chemical filtration;
Carbon, Purigen, Chemi-Pure, Poly-pads, zeolite, ion exchange resins, GFO and/or other phosphate removers, Cupra-sorb, etc., are all types of media used for chemical filtration.
Carbon, Purigen, Chemi-Pure, Poly-pads and ion-exchange resins all perform similar functions by removing dissolved organics compounds, chemical contaminants, odors, dis-coloration, metals, etc.
This is an important facet of the filtration schema as the more dissolved organic compounds you can remove prior to the biological section of the filter, the less workload on the bacteria colonies and as a result less nitrates present overall in the water volume.
Zeolite is more ammonia/chloramine specific.
GFO and a few other products are available but are specific to certain things, such as phosphate, but are still considered a chemical media.
The third and usually final section is the biological section. There are myriad products/things that can be used to fulfill this role, although the more porous, the better. It should also be of a size and shape that affords good water floe through it and helps preclude any bits of matter that may make it through from getting lodged amongst it.
I personally have grown to prefer Seachems line of Matrix bio media because it affords a tremendous amount of exterior surface area for aerobic bacteria for the nitrification process and an even greater amount of internal surface area with structures that are perfect for colonization by anaerobic bacteria for the de-nitrification process.
If set-up in this manner and with good mechanical filtration, and cleaning the mechanical section weekly or at least bi-weekly
(6 months is just asking for disaster) in theory the chemical portion will stay cleaner therefore remain effective for a longer period of time and the biological portion would rarely need cleaning.
hope that helps