Activated carbon is that black granular stuff obtainable from the local pet fish store. The product should be marked as suitable for marine use. Having said that, it isn’t an absolute necessity in a marine aquarium. It could be of use or not.
The protein skimmer does a very similar job to activated carbon, which is it removes dissolved organic matter, though it may be that the substances removed differ. In addition activated carbon could be of use after disease treatment in an aquarium to remove any residue of the treatment. Activated carbon also has the same downside as a protein skimmer and that is trace elements are removed from the seawater. Unlike the protein skimmer which just needs regular cleaning, activated carbon has a relatively short life (dependant on seawater content) and should be regularly changed. Once its useful life has finished, and this is when the carbon has adsorbed all it can, it is of no further use and should be replaced with new media. The appearance of the carbon will not indicate the condition.
Activated carbon can be placed in a canister filter, a very effective placement as there is plenty of seawater flow. Some aquarists place it in a bag and put the bag in an area of high flow – this is generally acceptable but there is always a question about how much seawater is actually going through the media. Seawater will take the easier route if possible which means it could be going round the bag.
As already said the carbon could be useful but it is not essential (essential as is, in my view anyway, a protein skimmer). There is a very simple test that could be done to indicate one reason why carbon is needed. Place a small white plate or saucer against one of the aquarium end glasses on the inside, or tape a piece of white paper on the outside. Now view the white object through the end glass at the opposite end of the aquarium. Is there any hint of a yellow tinge? If there is, it is termed ‘gilvin’ and indicates that carbon could be used or, if in use, it should be renewed. If carbon is in use and there isn’t a yellow tinge this does not indicate that the carbon is in good condition. The carbon should be renewed say every three weeks if in permanent use, dependant on the bio-load in the aquarium and the aquarist’s experience of it. This white object test is not an absolute indicator of the need or not for carbon, it is merely an aid.
A better way of using carbon is to use it periodically, say use it for a week then don’t use it for a further one or two weeks. The amount of carbon required for the week is small, and this carbon should be disposed of when the week is over. This way any unwanted substances in the seawater should be removed or partially so, and the amount of desirable trace elements removed should be minimized.
There is another check for the use of carbon, and that is not to use it at all. A check as suggested with a white object can be made after a time and, if all is well and an efficient protein skimmer is in use, if routine seawater changes are completed, if the livestock are healthy and thriving, then there isn’t a need for activated carbon.