Carbon is a very popular chemical filter media that is a must for most aquariums. It not only helps to keep water crystal clear but it also absorbs foul smells and unwanted chemical treatments. The carbon latches onto and stores these unwanted impurities until the carbon is removed. Carbon is very good for removing the yellow tanning than can occur in freshwater aquariums when bog wood is added as a decoration.
Swell UK sells Carbon as a loose substrate or impregnated in foam. Loose Carbon would be best to be combined with a filter media bag (if not included).