Its really not that hard. Some people even use legos as media in canister filters. There are three main categories of filtration:
1. Mechanical
2. Chemical
3. Biological (Most Important)
For simplicity, lets say you have an aquaclear.
The first layer is mechanical/biological, the sponge (RINSE/REUSE). It removes the particles and debris from the water and has some biological function because it hosts bacteria that destroy harmful ammonia. It does not have to be replaced ever unless its so far gone, you don't feel like washing it out again. Rinse it off with aquarium water once a month or as much as needed to allow it to do its job. You want to use aquarium water so that you do not kill the bacteria. Its ok to use high powered tap water every once in a while so you can clean it better.
The second layer is chemical, the carbon insert (REPLACE MONTHLY). If you have a planted tank as I do, you don't use it all the time. But most people use it. It removes toxins and odors. The carbon insert should be replaced every month. Don't wash it out and resuse it. If medicating the fish, take out the carbon while medicating because it will absorb all the chemicals you put into the aquarium.
OK now you have some options. The third layer can be the ammonia resin insert.(REPLACE MONTHLY) This removes ammonia. This should be replaced every month or two. If you have adequate biological filtration and do water changes enough, this insert is not necessary. You should have an ammonia test kit. If your ammonia is at zero you don't need this in your filter.
The next layer is purely biological (RINSE/REUSE). Aquaclear has an insert called "BioMax". They are small ceramic beads that people use in canister filters. They have tiny holes in them that give miles of surface area for benefitial bacteria. The sponge alone is great for biological filtration but this will increase you filtration capabilities. Pieces of lava rock are the same concept.
The final layer is optional as well. Many people use filter floss (REPLACE WHEN ITS DIRTY). In an aquaclear, I stuff filter floss into an old carbon bag (with the old carbon removeded out of course) this just holds the filter floss in place. The filter floss is not necessary but does a great job of catching the little pieces that the sponge may miss. Its used to "polish" the water.
* Peat. Peat is only used by people who want very acidic water. Peat will turn you water yellowish and can be messy. There are other ways to make your warer more acid like acid buffer and driftwood.
* I have mentioned the pre-packed inserts from aquaclear. You can buy empty filter bags or reuse empty ones when you dump out old carbon or ammonia-removing resin. In these bags you can put bulk carbon, bulk ammonia chips, a combination of carbon and ammonia chips so u just have to replace one insert once a month instead of two. And you can fill it with lava rocks or ceramic beads. The choices are many. You just really need mechanical and biological filtration.
