Agree with Tolak, carbon is a great tool to have on the shelf but is not usually needed for everyday operation.
Carbon (aka activated charcoal) is most useful for 3 things:
1) removing meds after they have finished their course
2) removing yellow tannins from bogwoods etc. if they are still leaching some after cleaning
3) removing the occasional organic smell, sometimes of unknown cause
The sponge and biomax media you mention should always be cleaned in *tank water*, not tap water. You could kill most or all of the beneficial bacteria if the tap water has strong chlorine or chloramines in it.
Aquarium filters have 3 main functions: Mechanical filtration, Chemical filtration and Biological filtration. Chemical filtration is optional and usually only used for special functions, such as discussed for carbon above. Mechanical filtration is the removal of debris and particles from the water and is the only function most beginners think a filter is for. Biofiltration is a fascinating process where two specific species of bacteria are grown, over time, in the biomedia of the filter and serve to clear the tank of ammonia and nitrite(NO2) via the process known as nitrification, part of the Nitrogen Cycle.
The media you put in your filter will often serve overlapping duties between some of these 3 functions, but each type of media may be better or worse at a given function, lending different strengths depending on how its placed and used in the overall filter design. Experienced aquarists often customize their use of various filter media.
Hope this helps a bit,
~~waterdrop~~
