Activated carbon is like a catch all for many of the problems that arise in the marine aquarium, and for good reason. If you’re ever having issues with an algae outbreak, discolored water, fish disease outbreak, or just things not looking so hot, chances are a fresh bag of carbon will “fix” it. The micropore structure within the carbon allows each particle to have tons of surface area, allowing it to absorb all sorts of chemical compounds or playing host to various other chemical reactions. This means that carbon will absorb medications, certain metabolic byproducts, heavy metals, and various other things. Despite all of its benefits, however, carbon won’t remove ammonia, so it won’t fix every problem. Additionally, carbon has been linked to certain fish diseases, specifically head and later line erosion in tangs, but it is still a great choice of filter media that could have a drastic impact on your tank.