WHY THE CARBON?

Chemical filtration is achieved with the use of activated carbon and ammonia absorbents.  (Zeolite, Ammo Carb, etc.) 

Each little piece of carbon detoxifies the water that passes through it by soaking up the different pollutants in the water, similar to the way a sponge soaks up water.  Ammonia, foul odors, colors, medications, and dissolved fish waste are absorbed as the water passes through it.  In order for the carbon to be effective, water must "pass through it", which is why we find it in places like the filtration units, where water is being forced through chambers containing carbon. 

Carbon, like a sponge can get "over full".  This is the reason for the change once each month, meaning total removal of the old carbon.  Much as a sponge that is over full, leaking water back out, carbon has the same potential with the toxins it absorbs.  The water in an aquarium is continuing to gather waste as long as it has inhabitants, therefore anything that is "leaked" back into the tank from old carbon, will add to the pollution, and make water conditions worse.  The use of carbon does not mean that we can eliminate the water exchanges each week.  Carbon can only absorb so much, the rest must be manually removed.