About Activated Carbon
It is often supplied as a black sponge or as small black beads which are used in filters to remove certain chemicals from the water.
Activated carbon is made of either coal, wood or nut shells (coconut, etc.) charcoal. For use in the aquarium, only coal or wood based charcoal is used; coconut based charcoal is used for cleaning chemicals from gases.
The sponge or granules are typically placed as the last stage in a filter, and manufacturers recommend replacing it after 2–4 weeks.
What does it do
Sellers of activated carbon are often vague about the benefits. Terms like 'clean' or 'polish' the water are often stated without actually saying what it does. Its purpose is to remove chemicals that colour the water, or create odors.
It removes some smelly organic gases like hydrogen sulphide from the water, absorbs chlorine from water, or if your tap water contains chloramine, it removes the chlorine and leaves ammonia behind. Also small amounts of chelated copper, mercury and iron may be removed and this may be of limited use.
Once very popular in the 1980s, its use is better understood in the 21st century and the benefits of constant use are now questioned.
It is probably overused by the beginner as it is commonly supplied with new filters and aquariums by manufacturers by default and users are often advised to replace it every month. This is probably a needless monthly expense if the tank is new and the tap water is already pre-treated with a water conditioner. Since carbon adsorbs organic chemicals on its surface, a good rinse and "grind" should refresh a batch for far longer than the manufacturer would like you to think.
- Users should remove the carbon from the filter if adding medical liquids or water conditioner liquids such as Amquel to the tank as it may absorb them.
- Constant use of activated carbon is not advised for aquariums with plants.
What it doesn't do
The material will not normally remove much ammonia from the water (It removes a little ). However there are several brands of AC on the market that add a special coating of zeolite which act as a ammonia absorber. These types should not be placed within a maturing tank not yet cycled.
How long does it take to work?
Activated carbon quality varies enormously, there are cheap and expensive products on sale all claiming to work wonders. But some studies (see The Krib Keslar article) show that in fact 90% of the adsorption work of a piece of carbon in a good water flow in fact occurs within the first 48 hours and after 100 hours (4 days) it can be removed. Higher quality carbon can effectivly remove contaminates from aquarium water for up-to four weeks 
Can I leave it in the tank
Activated carbon will not leak the chemicals back out regardless of how long you leave it in the water. However statements by companies like Seachem do say that all forms of activated carbon leak out amounts of phosphate. What differs between products is how much of this amount. This may contribute to the development of algae in your tank.
Of course the carbon foam rapidly becomes home to your biological nitrifying bacteria filter. In a new tank in the middle of nitrogen cycle the carbon should never be removed for at least 40 days to ensure the cycle is fully established.
Can I recharge it
Yes and no. Once activated carbon has adsorbed chemicals, it should be thrown away.
It is possible to heat activated carbon to 200°C for 30 minutes. But this will only destroy the organic elements it has absorbed. Any metals will still be present.
It is possible to get a bit more life out of it by washing thoroughly (in water only) and physically grinding it a bit. This removes bacterial colonies that have grown and block the water's access to the carbon, and also opens fresh surfaces on the carbon particles.
The quality of the carbon is proportional to the of surface area exposed to the water flow. Higher quality carbon has a smaller structure. It is possible to crush lower quality carbon granules to produce more surface area. The resulting fragments should be washed thoroughly to remove any carbon dust that could be blown into your tank.
The following video details an experiment showing how quickly lower quality carbon is used when compared to the slightly more expensive high quality carbon.
- Absorption of ammonia on activated carbon from aqueous solutions
- Carbon and aquariums - Aquatic Community
- Leo Morin, Ph.D. (Seachem) talks on Activated Carbon, it's uses and how to choose a good brand
- The Krib on Activated carbon
- Tom Griffin article (this is an archived link, so it is slow to load. But worthwhile reading)
- Analysing the Effectiveness of Brita® Water Filters - A study on the effectiveness of a domestic water filter using activated carbon
- Activated Carbon by Timothy A. Hovanec
- Revisiting Activated Carbon by Timothy A. Hovanec
- Granular Activated Carbon, Part 2: Modelling of Operational Parameters for Dissolved Organic Carbon Removal from Marine Aquaria
- Overview of Activated Carbon in the Marine Tank by James R. Layton, chemist for Aquarium Pharmaceuticals.