Activated carbon is a very useful tool for any aquarium, but you have to understand how it works and how to use it. I like to think of activated carbon as a kitchen sponge.
1. Like a sponge it soaks things into itself and holds it there
2. Like a sponge, once full, whatever was trapped just leaks back out
3. Like a sponge, its nice to have to clean up little mistakes
So carbon acts like a sponge soaking up whatever, good or bad, in the water and trapping it there safely until it is full. What will it trap? I have never been able to obtain a good list, but I will list some of the things I know.
Carbon is GOOD at trapping…
3. Some heavy metals (Mercury, Gold)
Carbon is BAD at trapping…
2. Carbon Dioxide
3. Carbon Monoxide
4. Salts/Salt Solutions
8. Iron (not the kind you use as a fert)
Again, I have never found a full list and not all carbon is made the same or have the same abilities. Some activated carbon sold for aquariums is just plain carbon (charcoal) and will do nothing. Also, the amount most people have is useless. It is recommended to use as much as 1 pound for every 10 gallons. Makes that little packet on your hang-on filter look kinda pointless huh?
So you have your sponge and you have some idea of what it is going to soak up. We can see it can get rid of some real nasty stuff. However, we can also see that it completely ignores some of the most common nasty stuff found in an aquarium. So what good is it?
Back to the sponge. It’s good to have if a mistake is made. Did your wife just spray a spider with insecticide that was hanging out over your tank? Grab the carbon (and some fresh water to do a water change). Worried about harmful gasses under the substrate while moving things around? Grab the carbon. You get the idea.
For regular filtration, it is my opinion that it is a poor choice. It does have a lot of surface area, but it gets full and leaks out whatever it has trapped. Better to use that space with bio media like floss, sponges and rings. I would rather have it on hand for emergencies or when the tank has aged and the water gets to looking a little yellow.