How does Activated Aquarium Carbon help my fish?

Activated Carbon () is a  substance which is used in the aquarium to remove chemicals from water.
Photo provided by Flickr

Although not a scientifically measured indicator of the quality of activated carbon, if a package of activated carbon contains minimal amounts of dust, that's a definite plus. The dust does not pose a specific threat to health, but if enough is present it can be a (likely temporary) irritant to your tank inhabitants, and will also make the water in your aquarium appear cloudy.
1 x BOYU AC-500 Aquarium Activated Carbon
Photo provided by Flickr
Of the three types of filtration used in the reef aquarium hobby today, biological, , and chemical, it is chemical filtration that is possibly the least understood. This post is all about activated carbon, the most popular type of chemical filtration used in aquariums. 1 x BOYU AC-300 Aquarium Activated Carbon
Photo provided by Flickr1 x BOYU AC-150 Aquarium Activated Carbon
Photo provided by Flickr6 grams (.21 ounces) of Lignite Activated Carbon has the surface area of a football field, so a little goes a long way in aquarium use in particular.
Photo provided by Flickr
Activated carbon for the aquarium is a form of carbon that is usually made from bituminous coal, lignite or wood. It is often abbreviated as AC on the forum. The primary use of activated carbon, or AC, is to filter the aquarium water of foul odors, yellowing compounds (DOC) and to remove medications from the water column. Some hobbyists use it regularly and some don't use it at all. Some swear at it and some swear by it. I'll try to give you the basic info to get you started and you can decide for yourself whether or not you think it's something you will want to use.Granular Activated Carbon (GAC) is frequently recommended for inclusion in aquarium filters. It is usually manufactured from coal, and occasionally from wood such as coconuts. It is recommended for use to remove unwanted colors and aromas from the aquarium. And it does work very well for this purpose. However, there are also some problems with its use.Activated carbon comes in various shapes and sizes and not all activated carbon is equal. The most common shapes are granules and pellets. There are powder forms as well, but those are not used in aquariums. Generally speaking, the more porous and lighter the carbon the more surface area it has for adsorption thereby making it more effective at filtering the water. Many tap water filters are made to use activated carbon and it does a great job at removing odors from the water. This can apply to your aquarium as well.However, there are some potential problems you need to be aware of. Phosphorus, for one. Some brands of Granular Activated Carbon contain phosphates left over from the manufacturing process, and can add phosphates to the water column. This encourages unwanted algae growth. Normally, you would never want to add phosphates to your water because they tend to stimulate the growth of unsightly algae. If you notice algae growing in your aquarium and you are running Granular Activated Carbon in your filter, you might want to try removing the carbon to see if that doesn’t solve the problem.GAC works by a process called aDsorption. (Notice the D where most authors use a B.) This means that large organic molecules cling to the carbon, and they are not released back into the water. The process works best when water flows slowly through the carbon, rather than being blasted through at a brisk rate. Slow-moving water gives the carbon more time to work. It doesn’t take a lot of carbon to do the job, either. 3 or 4 tablespoons of Granular Activated Carbon are plenty for a 55 gallon aquarium. It does have a finite life, and will usually clog up in about 30 days. So, when the filter is serviced and cleaned (you DO service and clean your filter… right?), the old carbon should be discarded and fresh put in its place.About the best advice I can give you if you've never used activated carbon is to try it out for yourself. If you have an aquarium odor problem or yellow water issue, use AC for a couple of days and see how it works for you. If you have a tank full of corals, especially lots of different coral species, try running a "good" acid washed activated carbon for a period of time and see what kind of results you get. You don't know until you try.