This has been interesting to me. My filter, an Aqua Clear Power filter, says to switch out charcoal pads each month, the sponge pad every 2 months and the biofilter every 3 months. It said never to switch out more than one filter at a time (kind of hard NOT to do it If your switching out a charcoal filter monthly). Frankly I was never that concerned about my sponge filter but was very concerned about dumping the biofilter in the trash. After listening to all this I'm starting to wonder if it's just a ploy to get you to replace filters all the time. So whose instructions should I follow? Aqua Clears or a group of experienced aquatic hobbyists (actually professionals in some cases)? I don't care about voiding the warranty - the filter works great but is a pain in the neck to keep level (they give you a little plastic part to stick in a hole behind the filter and it falls out constantly, if it didn't also adjust the flow I'd just superglue it on - I may yet).
There are two aspects to this issue. First, on the carbon media. Carbon adsorbs (not absorbs, but adsorbs with the "d") substances from the water. As it does this, it becomes clogged to the point where it will no longer adsorb anything. Before or at that point, it must be replaced; there is no method to somehow "clean" carbon for re-use, once clogged it is clogged. The amount of substances the carbon removes determines its longevity. If left past this point, it no longer adsorbs but will obviously function solely as ordinary biological media as the bacteria will colonize the carbon just as any other media.
The biological media such as the sponge/foam material will collect bacteria and debris and must be rinsed regularly; this media does not need replacing until it no longer functions, i.e., it is literally falling apart. Water has to get through this media, not around it, and not be blocked. Rinsing such media during the weekly water change (with sponge filters, etc) is all you need. Some sponge breaks apart faster than other sponge. I have over the past decade noticed that my original Hagen Elite sponge filters began to lose shape and fall apart after a couple years, whereas the slightly larger sponge filters I ordered online (couldn't find the Elite anywhere) have lasted far longer, some five or six years now, and still are as good and solid as new.
Coming back to the carbon filtration on another aspect. Carbon filtration is chemical filtration, which applies to any filter media that somehow changes the chemistry/makeup of the water. Mechanical filtration merely removes suspended particulate matter to keep the water clear. Biological filtration involves the nitrifying bacteria and all filter media will function as biological filtration. The chemical is the third type, and one that is usually not necessary. Carbon removes organics which are primary plant nutrients, so in planted tanks you should not ever use carbon filtration because it is removing plant nutrients. In tanks with no live plants, carbon should not really be necessary but it can't hurt. Carbon can be useful to remove medications after a treatment, though water changes usually do this just as easily.
