This article first appeared in the April 1990 issue of Aquarium Fish Magazine
Glitter Lines Part 3: Chemical Filtration

by J. Charles Delbeek M.Sc.

The topic of chemical filtration is described in just about every 
textbook written on marine aquarium keeping, yet few people really 
seem to understand its capabilities, limitations and applications. 
There are numerous forms of filtration that could fall under the 
category of chemical filtration depending on their mode of operation. 
For the purposes of this article we will limit our discussion to the 
common forms of chemical filtration used in reef systems namely 
activated carbon, foam fractionation (a.k.a. protein skimming), 
molecular adsorbants and ozone.

Due to the various biological processes which can occur in an aquarium 
a build-up of organic substances takes place. They are referred to as 
organic because they all contain the element carbon in their chemical 
composition. The list of these things is quite lengthy and includes 
such goodies as amino acids, proteins, phenols, creosols, terpenoids, 
fats, carbohydrates, hydrocarbons, plant hormones, vitamins, 
carotenoids and various organic acids such as fatty, acetic, lactic, 
glycolic, malic and citric (deGraaf, 1981; Moe, 1989). Fortunately for 
us, we generally lump all these things together under the all-
encompassing term dissolved organic carbon (DOC). These organic 
substances can have various deleterious effects on the aquarium 
inhabitants ranging from reduced growth, to reduced disease 
resistance, to metabolic stress. In some cases these organics are 
mineralized, by bacteria present in the tank, into ammonia. The 
ammonia is then oxidized by the nitrifying bacteria we are all 
familiar with to the final product, nitrate, which then tends to 
accumulate in the aquarium. Unfortunately, many of these organic 
substances are not mineralized and tend to build-up in the aquarium. 
That is why water changes are usually advocated. Many people think 
that water changes are designed to lower the nitrate concentration. 
While this may occur to some extent, the real reason is to lower the 
DOC content of the water. Since nitrate and DOC concentrations are 
often directly related, and nitrate is easy to measure, it is used as 
a yard stick to determine when to make a water change. 

The thinking then goes that if we could remove much of the DOC before 
it accumulates or becomes converted into ammonia, we could reduce the 
need for water changes, lower the stress on our filter, lower nitrate 
levels and improve the growth and health of our organisms. These then 
are the primary reasons behind the use of chemical filtration. 

At this point I would just like to clarify that I am not saying that 
if you use chemical filtration, water changes will no longer be 
necessary. This is, of course, not true. First of all no method of 
chemical filtration is 100% efficient and many substances are 
difficult to remove by chemical filtration. Secondly, water changes 
provide other benefits besides removing and diluting DOC, such as 
providing pH control, trace elements and calcium. Even in the best 
maintained aquariums, the affects of a water change on the inhabitants 
can be quite stunning. What chemical filtration WILL do for you is 
help maintain a much lower concentration of DOC in your tank. This 
becomes extremely important when dealing with invertebrates such as 
hard corals. 

Activated Carbon
Many of us are familiar with the use of charcoal in the old cornerbox 
filters of our freshwater days. These usually consisted of small, 
shiny, irregularly shaped pieces of bone or wood charcoal. This type 
of carbon is not really suitable for use in marine aquariums and has 
been replaced by "activated" carbon. In this form the carbon has been 
subjected to extremely high pressures and temperatures to drive out 
all impurities and gases leaving behind extremely porous and pure 
grains of carbon. Particle size, type of gas used, activation 
temperature and, in some instances, inorganic salts of zinc, copper, 
phosphate, silicate and sulphate added before activation, provide 
carbon with specific adsorption characteristics (Moe, 1989). Therefore 
activated carbon can be tailored to the specific type of impurities 
that one wishes to remove. By creating such extremely porous 
structures within the carbon grains we have, in effect, created a 
gigantic sponge that can absorb many impurities from the passing 
water.

Activated carbon will remove a wide variety of organic molecules by 
simply trapping them in the carbon pores (absorption) or by chemically 
bonding them (adsorption). Adsorption relies on the fact that many 
organic molecules are polar in nature. This means that the two ends of 
a molecule differ in their affinity for water. One side is repelled by 
water and is termed hydrophobic ("water hating") while the other end 
is attracted to water and is called hydrophilic ("water loving"). When 
a polar molecule comes close to a polar surface such as GAC, they will 
become attached to each other, effectively removing the molecule from 
solution. Moe (1989) gives a detailed discussion of the properties of 
GAC and the factors that determine its efficiency so I will not go 
into great depth on these topics. 

Perhaps the most common mistake made in using GAC in trickle filters 
is its placement. When GAC is used in outside power filters the 
instructions always tell you to place the carbon between pads of floss 
and to make sure that all the filter water flows through it. We all 
know that right? Makes sense doesn't it? Then why does one always see 
bags of GAC placed in the sump of trickle filters such that the 
majority of the water passes around it and not through it? The sump of 
the trickle filter should be designed so that all the water must flow 
through the GAC chamber. If this is not possible then there are a 
couple of options available. One is to hook-up a secondary canister 
filter to the sump, fill it with GAC and run this through the sump. 
The other is to build an in-line contact chamber in the return. This 
consists of a section of PVC pipe with hose fittings at both ends. 
The pipe is filled with GAC and is placed into the return line so that 
all the water returning to the tank passes through it. Thiel 
(1988,1989) and Moe (1989) describe the construction and placement of 
such units. Whatever method you decide upon, make sure that the 
majority of water flows through the GAC or else it will not be very 
effective.

One of the most common questions concerning the use of GAC is how much 
to use and how often it should be replaced. These questions are very 
difficult to quantify simply because no two systems are identical. 
Differences in bioload and the type of organisms being kept greatly 
influence the type and amount of DOC produced. For example, aquariums 
filled with marine algae will produce a greater variety of DOC than 
systems with very little algal growth. Thiel (1988) recommends using 
36 ounces of GAC per 50 gallons while Wilkens (1986) recommends 500 
grams per 100 litres, which is roughly equivalent. Although this does 
seem excessive one could use this as an upper figure and work 
downwards. The real indicator will be the inhabitants of the system 
and the colour of the water. Too many aquarists today are turning 
towards technological wizardy to maintain their aquariums. People are 
constantly talking about ozone, redox potential and carbon dioxide 
systems when the majority cannot correctly identify their tank 
inhabitants or don't fully understand what pH is. The occupants of our 
aquariums are far more sensitive to water chemistry than any meter and 
if one spends more time watching them instead of the flashing lights 
on your instruments you will be more in-tune with what is really going 
on in your tank. 

For the same reasons mentioned above, it is difficult to recommend a 
specific time period after which the GAC should be replaced. However, 
various authors have stated that GAC should remain active for 5-7 months 
before needing replacement (Moe, 1989; Wilkens, 1986). Generally the 
presence of yellowing substances in the water can be used as a guide 
to determining if your GAC needs replacing since these are easily 
removed by GAC and will start to accumulate when the GAC begins to 
lose its activity. Moe (1989) describes the following method. Obtain a 
strip of white plastic and colour one half a faint yellow with a 
marker. Place the strip in the water and observe from a distance. When 
you can no longer distinguish the yellow half from the white half, 
your water contains yellowing substances and you should replace your 
GAC. Since GAC is a very porous substrate, nitrifying and denitrifying 
bacteria will quickly colonize it. If you use large amounts of GAC 
then replacing all of it every six months or so could lead to other 
problems. If might be wiser to replace 30% with new carbon and rinse 
the remaining 70% with seawater (Wilkens, 1986). Put the new carbon in 
a separate bag and place it in-front of the old carbon. This will 
preserve a large amount of the bacteria that have colonized the GAC. 
Wilkens (1986) recommends also, that if GAC is to be added to an 
established aquarium, it be done gradually say 20 grams per 100 
litres, added every month until the figure of 500 grams, mentioned 
previously, is attained. The sudden addition of a large amount of GAC 
into an established aquarium can remove such a large amount of DOC 
that the animals may become severely shocked.

There are numerous brands of GAC being marketed today, some of which 
have fancy names such as research grade. Unfortunately, not all GAC is 
created equal and the levels of efficiency and quality vary greatly. 
The grains of GAC should be small, dull black in colour and as 
dustless as possible. Recent measurements of GAC filtered aquarium 
water have shown that certain brands of GAC will actually add phosphate 
to the water, which is exactly what we are trying to avoid (J. Sprung, 
personal communication)! Another problem is that as the GAC ages, some 
of the substances it has adsorbed and absorbed, can be released back 
into the water (Thiel, 1988,1989). However, if you change your GAC on 
a regular basis, you should be able to avoid this. A final caveat 
concerning GAC is that, along with the other forms of chemical 
filtration to be mentioned in this article, it indiscriminantly 
removes substances from the water, including some useful ones. 
Therefore, regular water changes take on an added importance when 
chemical filtration is present. 

Foam Fractionation
A method of chemical filtration which has been around for decades but 
seems to only now be catching on is foam fractionation (protein 
skimming). Foam fractionators consist of a column through which a 
very fine mixture of air and water is pumped. If any of you have been 
to the beach you may recall seeing foam along the beaches. This foam 
is produced by the action of the waves which combines air, water and 
certain polar organics to form a stable foam. A foam fractionator 
works in a similar manner and by collecting the foam, proteins and 
other organics are removed before they are mineralized into nitrogen 
containing compounds and other toxins which is very beneficial to the 
health and maintenance of the system. Of the various chemical 
filtration methods available, only foam fractionation completely 
removes most organics before they begin to break down (Moe, 1989). The 
list of substances removed by skimming include, amino acids, protein, 
metals such as copper and zinc complexed with the proteins, fats, 
carbohydrates, phosphate, iodine, fatty acids and phenols. A more 
detailed discussion of foam fractionators, their operation and 
construction, will appear in a future issue of AFM.

In my opinion, a foam fractionator is an indispensable piece of 
equipment for the marine aquarium, doubly so in a reef system. Foam 
fractionators have been in use in European aquariums for years and are 
often the sole form of filtration used. Obviously this cannot be 
achieved with the smaller, internal skimmers sold for years in North 
America, but by larger, external models. Such models are becoming 
common in North America, some of them European-made with many more 
being North American built. Although the majority of skimmers sold 
today are driven by wooden airstones, some models are available that 
incorporate a Venturi design. A Venturi skimmer uses a strong water 
pump and a small air inlet which creates a suction that forms a fine 
mixture of air and water in the skimmer. Such devices are more 
powerful and require less maintenance than the standard wooden 
airstone driven models, they can also be made smaller. 

There are a few things to keep an eye on when using a protein skimmer. 
First of all, the continuous removal of small amounts of seawater by 
the skimmer, along with replenishment of evaporated water with 
freshwater, can lead to a gradual lowering of salinity. Therefore, the 
periodic addition of seawater may be necessary to maintain the desired 
level of salinity. Secondly, efficient skimmers can remove some trace 
elements and regular water changes or the regular addition of trace 
elements may be necessary. Finally, the addition of certain buffers 
and molecular adsorbtion filter pads can cause the skimmer to foam 
excessively. In this case, the best action to take is to turn down the 
skimmer for a day and then gradually restart it.

Molecular Adsorbtion Filters
This form of chemical filtration is a relatively new addition to 
marine aquariums. At the moment the hobbyist market is dominated by a 
single product, Polyfilter(TM), marketed by Polybio Marine Inc.. This 
type of filtration consists of various styrene or acrylic polymers 
that selectively adsorb polar organics and nitrogen containing 
compounds onto their surface (Moe, 1989). Some (Thiel, 1988) claim 
that these products will remove phosphate from the aquarium. Although 
I did not measure phosphate in my own aquarium, I have noticed that 
the growth of red microalgae visibly slowed after the addition of such 
a filter pad. If the ionic interference caused by seawater can be 
overcome, and molecular adsorbants become more specific, we should see 
a proliferation of such filter products in the future. For example, 
products that selectively removed nitrate and phosphate down to the 
parts per billion level would be especially useful. As with GAC, 
molecular adsorbants should be situated so that water is forced 
through the medium, not around it. It is not known whether long term 
use of such filters will lead to trace element depletion. 

Ozone
Ozone is a naturally occurring gas in the upper atmosphere, where its 
UV absorbing properties have been given wide exposure in relation to 
its recent depletion caused by chlorofluorocarbons (CFCs). Ozone is a 
powerful oxidant since it consists of three atoms of oxygen (O3) and
readily releases the extra oxygen atom to become the more stable, and familiar,
O2. It is this property which we utilize in the aquarium by using
its oxidizing ability to breakdown organics and nitrite. Unfortunately, other
products such as hypochlorite and hypobromite, can also be produced,
that can damage delicate invertebrates and fish gills (Moe, 1989). 

Ozone is generally used in conjunction with a foam fractionator or 
pressurized air reactor. Ozone is mixed with air and is introduced 
into a contact chamber. The ozone-air mixture mixes with the aquarium 
water and organics are oxidized. The effluent is then passed through a 
container of GAC before being returned to the aquarium to remove any 
residual ozone and any harmful by-products that may have been 
produced. Used in conjunction with a redox controller, precise control 
over a system's redox potential can be obtained. Stated simply, redox 
potential is the ability of your water to oxidize and/or reduce 
substances in the water. Measurements of redox potential in the ocean 
vary from 350-400 millivolts (Moe, 1989) to as low as 160-190 
millivolts (Wilkens, 1986). However, caution is advised in any 
comparisons due to differences in measuring conditions, technique and 
equipment used. Recommended redox levels range from 375-450 millivolts 
but each aquarist MUST go by the appearance of his/her own aquarium. 
Differences in probe placement, frequency of cleaning, bioload, etc. 
all affect redox readings; it is not so much the value that is 
important but the appearance of your aquarium inhabitants. Once you 
reach a redox level at which you feel your tank looks best, than that 
is where you keep it, don't try and strive for levels you "hear" are 
recommended. 

Although many periodicals and books relate that ozone use in Europe 
(see Moe, 1989) is quite common, I have read numerous articles from 
both Germany and Holland advocating against the use of ozone. It is 
unclear what exactly the basis of their opposition is but the main 
criticisms appear to be that ozone is not necessary to have a 
successful tank, its use will cause problems in the long run and the 
various by-products produced are potentially dangerous to the 
inhabitants (Hebbinghaus, 1989; Stüber, 1989; Wilkens, 1986). Yet I 
have seen many beautiful aquariums here in North America that use 
ozone, in conjunction with redox controllers, on a continuous basis. 
One thing that I, and others, have noticed though, is that reef 
systems that use ozone tend to run at higher nitrate levels than reef 
systems that do not. This may be a reflection of the increase in 
nitrate production caused by the oxidation of nitrite into nitrate by 
ozone and/or some inhibitory ozone affect on denitrification. Finally, 
Stüber (1989) reports the growth of over 11 species of reef building 
hard corals without the use of ozone in an aquarium with a measured 
redox of 180 mv!

Both Moe (1989) and Thiel (1988,1989) go into much more detail on 
redox, ozone and their application to reef systems and I urge you to 
consult these references if you would like further information.

Whether used singly or in conjunction with other forms of biological, 
chemical and mechanical filtration, it is safe to say that chemical 
filtrants are an important component of reef aquarium filtration 
systems and should be used by every aquarist.

In the next installment of this series we will take a look at one of 
the most controversial elements in the reef tank ... lighting.


References
deGraaf, F. 1981. Handboek Voor Het Tropisch Zeewateraquarium. A.J.G.
                 Strengholt Boeken, Utrecht, The Netherlands.

Hebbinghaus, R. 1989. De stand van de zeeaquariumhouderij in de Duitse 
                 Bondsrepubliek. Het Zee-Aquarium 39(11):199-203.

Moe, M.A. Jr. 1989. The Marine Aquarium Reference: Systems and 
                 Invertebrates. Greenturtle Publications, Plantation, 
                 FL.

Stüber, D. 1989. Wat doen ze daar in Berlijn eigenlijk? Het Zee-
                 Aquarium 39(11):199-203.

Thiel, A. 1988.  The Marine Fish and Invert Aquarium. Aardvark Press,
                 Bridgeport, CT.

Thiel, A. 1989.  Advanced Reef Keeping I. Aardvark Press, Bridgeport, 
                 CT.

Wilkens, P. 1986. Niedere Tiere - Roehren-, Leder und Hornkorallen.
                 Engelbert Pfriem Verlag, Wuppertal.