This is probably the least understood form of filtration that is required in fish-only as well as in Reef tanks, even more so in the latter.
Mechanical filtration removes particulate matter of varying sizes; biological filtration removes ammonia and nitrite by means of bacterial processes, and converts it to nitrate; chemical filtration removes yet other types of compounds, some of which are not normally removed by the former, some of which are not removed by protein skimmers either (see next section).
The best known form of chemical filtration is activated carbon, others include molecular absorption filters (e.g. Poly filters), resins (e.g. HyperSorb), and special compounds meant to selectively remove certain elements from the water.
What kind of compounds are we talking about? Besides the undesirable elements already mentioned, e.g. free floating material, nitrogen breakdown intermediates, seawater absorbs many noxious elements from the surrounding air (e.g. nicotine, tar, fumes, perfume essence, volatile organic compounds etc..), chemicals that, when they accumulate, can stress the tank life considerably.
Besides these, many chemical compounds are formed in the aquarium and are not removed by the filters already mentioned. Included in this category are, for example, yellowing matter (gilvin), organic acids, heavy metals that may come in with the tap water, pesticides in very small quantities (ppb's), etc.
The principles of chemical filtration are based on molecule adsorption, and on ion affinity, allowing certain compounds that we add to the filtration system to remove unwanted elements very efficiently. Because we do not "see" the filter at work, certainly not as much as we can determine what a mechanical filter is doing for example, these type of filters can be extremely beneficial, especially if the material used is replaced on a regular basis.
I conducted many experiments along the lines of chemical filtration, using an assortment of chemical filtering materials. All work, some much better than others. Two types in particular are very familiar to most hobbyists : activated carbon and Poly Filters. Resins and other compounds are used less frequently, except perhaps for the "X-nitrate" compound from Thiel-Aqua-Tech, which is very specific for Nitrate ions, and can lower the total NO3 in seawater to the 2-5 ppm range in a matter of 7 to 10 days. Unlike carbon, this product cannot be regenerated. Then again, regenerating carbon is not recommended anyway, as Hobbyists do not really have the equipment to do so efficiently.
Many types of carbon are sold in Pet Stores. Unfortunately the Hobbyist has no way of knowing which ones are good, bad or better. Some brands are merely charcoal (usually shiny) and are very ineffective at removing noxious compounds. You should definitely avoid them in Reef Aquariums.
Other types are "activated", a process that substantially increases the porosity of the carbon, and as a result greatly increases the total active surface area, usually depicted in drawings as an intricate path of holes, and channels in the carbon, allowing it to be much more effective at adsorbing compounds that need to be removed from the Reef tank's water. The amount of pores and channels is what is of interest to us.
Activated carbon has a maximum adsorption capacity, which is the reason that it should be replaced from time to time. How frequently is hard to determine, because it is dependent on the quality of the water that is flowed through the carbon. Note that we said flowed "through" and not over. Your activated carbon will work much better if water is actually forced to go through the amount used, rather than just flowed by, and over it. More contact between the activated carbon and the water obviously results in a more efficient cleansing process.
One problem that frequently occurs in saltwater tanks with a heavy organic load, is that the outside of the carbon becomes quickly coated with organic matter and slime, preventing further uptake and adsorption of unwanted compounds from the water.
How much carbon you should use, if you decide on this method of chemical filtration, is open to much debate. Some advocate small quantities of only a few ounces per 50 gallons of water in the system, to quantities that are 7 to 8 times larger.
Our recommendation is that if you use a good quality brand, you can safely follow the manufacturers' recommendations. Perhaps the most frequently recommended type is Chemi Pure from Dick Boyd Enterprises. It has been around for along time, and has obviously proven itself to many Hobbyists. We use our own Tech-Coconut-Shell brand.
Activated carbon is used to remove DOC (dissolved organic compounds), yellowing matter, heavy metals e.g. copper and iron (to some extent), airborne pollution, trace elements, intermediate protein breakdown products, and so on. Quite an amount of elements indeed. It is, therefore, an efficient chemical filtration material.
Since it removes trace elements as well, it is important to perform water changes from time to time (4 to 5 percent a week is my usual recommendation) to replenish such needed elements, or add one of the commercially available trace element mixes (e.g. Reef Elements from T-A-T, CoraLife, HW Trace Elements, Tropic Marine, etc.)
When using activated carbon, keep the following in mind :
Smaller granules, or pellets, will do a much better job, because of a higher total surface area, meaning greater porosity.
If only small amounts of activated carbon are used, you will need to replace it often, perhaps as much as every 2 to 3 weeks.
Rinse all activated carbon before using it, to remove fine material and dust, that will otherwise end up in the tank and on the rocks. This black dust is hard to remove, and makes the tank look very unsightly, besides being noxious to fish and invertebrates.
Ensure that a good "through" flow takes place. Do so by placing the carbon in a compartment of the trickle filter through which all the water has to flow, or better still :
place the activated carbon between layers of polyester floss filtering material, in a good quality canister filter. The new Eheim 2200 series canisters are highly recommended as they offer good flow and move quite a bit of water. Magnum and Fluval are other choices.
Because good quality activated carbon is expensive, it is important to pre-filter the water that goes through the canister, or the compartment in the filter, to prevent the carbon from becoming coated with fine particulate matter, and acting as a mechanical rather than chemical filter. This ruins the efficiency of the activated carbon fast.
Test the carbon that you plan to use for leaching of phosphate. Soak some carbon in a little tap water, or aquarium water, wait about 6 hours. Now take a sample of the water and test it for phosphate content. You will be surprised to find the number of brands that actually leach phosphate into the aquarium. Such types of activated carbon are of course totally unacceptable for use in Reef Aquariums. J. Asero (1989) evaluated many types of carbon, and found only few that did not leach carbon in the water. For a more detailed review we refer you to Marine Reef, the newsletter, Volume 2 Number 20.
Some carbons may also leach sulfate, and metals, into your water. Make sure you buy top quality carbon and you will avoid these problems. Coconut shell carbon is probably the best, but also the most expensive. In a Small Reef you should not take any chances.
Make sure that you change the activated carbon regularly. Many types of carbon are known to leach some of the matter they first adsorb back into the water, once they have exhausted their ability to remove matter from the water. Chemically speaking what may be at work here, is ion displacement , with the ones for which the carbon has more affinity displacing ions for which there exists lesser affinity. The displacement results in these ions ending back up in the water (Thiel, 1989).
Exhausted activated carbon will soon start acting as a small biological filter and will also, as time goes by, act as a mechanical filter. This is not desirable because you have no control over what compounds leach back into the water. Spent activated carbon should, therefore, be removed and replaced. You can tell whether your carbon is still "working" (removing compounds from the water) by checking whether your water is yellowing. Make a very faint yellow mark on a small piece of white plastic, submerse it and look at it from about 12 to 18 inches away. If you cannot see the difference between the very faint yellow and the white, your water is obviously slightly yellow, and your carbon needs changing.
Some lesser quality types of activated carbon may alter the pH too much. Usually they will raise it. Sometimes considerably. Acid washed carbon will not.
Fresh activated carbon will lower dissolved oxygen levels for a short period of time, and may as a result lower your redox potential as well. This is however short lived.
Soak new carbon in warm water to remove gas bubbles trapped in the pores. This will improve the efficiency of the carbon substantially.
When in doubt about the freshness of your carbon, do not use it, or if it is in your filter already, change it.
You may also want to filter the air going into your trickle filter by running it through a small column of carbon first. This will remove airborne pollutants. An old air dryer that you are not using can be filled with a good brand of carbon and place in-line with the air flow. Some units are commercially available (e.g. T-A-T).
Carbon removes organics from the water, to some extent anyway, but does not do away with the need to use a properly sized protein skimmer on your system as well. All you get is some degree of polishing of the water, not total removal. Refer to the section on protein skimmers for more details.
Pulverized activated carbon offers the largest surface area of all carbons. It is however not as easy to use, and needs to be set up with a diatom filter, or low micron filter, at the same time. This will result in the need to change the carbon frequently, as diatom and low micron cartridges, or special very low mesh bags, will of course plug up faster, as they trap very small particulate matter as well.
Of all the activated carbon types I have used myself, I prefer a granule, about one eighth to three sixteenths of an inch in diameter. The granules must be "dull", not shiny! Of course, that is the kind my company sells, as it is the one that I have found works best.
Do not overlook the need to add carbon to the system if you are using ozone. Best is to flow the water coming out of the skimmer over a little activated carbon, before it can re-enter the main water stream back to the aquarium. Trickle filters that have compartments built-in that allow you to do so are, therefore, to be preferred.
Although at some time I was convinced, based on experiments, that in the case of activated carbon "more was better", I have since changed my recommendations. I now recommend approximately one half pound of activated carbon of very high quality per 75 gallons of water, with the stipulation that the activated carbon must be changed every 5 weeks, regardless of how well the tank may be doing. This will ensure that the carbon is always removing pollutants from the tank.
You can safely mix several forms of chemical filtration. For example, you can use activated carbon and resins, or activated carbon and molecular absorption pads (Poly Filters) or disks (PMA discs from Poly-Bio Marine) . The latter of the two is the better.
It may appear to you, that selecting carbon has become a little more complicated than it used to be. Indeed it is. Unfortunately the better qualities we refer to are hard to come by. Knowing all the problems associated with activated carbon, my own company obviously sells only top quality coconut shell, acid washed to ensure a stable pH.
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