TERMS
First, a little vocabulary. Not
everything sold in stores for use in aquarium filtration
is activated carbon even though it might be black
and it might even have the name activated carbon,
or a close variation, on the box. Boxes of cheap black
shiny bits of material are probably anthracite coal
and have little to no adsorption capability. While
there are standards for activated carbon, they are
more informational than legal; companies do not have
to meet certain criteria to label their product activated
carbon or similar. Activated carbon (or just carbon
for this article) can be made from almost any organic
based material such as wood, coal, peat, coconut shells,
and many other things. Activating the material, which
increases its internal surface area and so to adsorption
capacity, it is done through special processes which
involve heating the material to nearly 2,000 (F in
a controlled environment. A substance becomes activated
carbon by undergoing the controlled process of increasing
its internal surface area, so as to increase its ability
to remove certain substances from the water (or air).
The process of the carbon removing a substance from
the water is adsorption, not absorption.
Activated carbon removes a class
of material called dissolved organic compounds (DOC).
These compounds are carbon based and are produced
by many different processes in the aquarium. The two
major categories of organic substances that carbon
removes are tannins and phenols. Tannins give the
water a yellow-brown color. They are mainly produced
by the breakdown of plant material but there are other
sources too. Phenols give a fish tank that characteristic
"fishy" smell. Carbon can also remove chlorine
and some heavy metals through other processes.
WHAT HAPPENED TO THE COCONUTS
Perhaps the biggest difference
between 1993 and 1998 is that there are very few coconut-based
carbons available now. In the late 1980's and early
1990's, carbon made from coconut shells was introduced
to the hobby. It was the subject of much marketing
hype and every magazine had advertisements with many
claims about its superior ability. Today, the ads
have disappeared as have most coconut carbons from
the store shelves.
It took awhile, but the drawbacks
of using coconut carbon (which were detailed in the
1993 article) in the aquarium finally merged with
the markets forces, and sales of this type of carbon
dropped. The reason that coconut carbon failed is
mainly that coconut carbons have the wrong pore size
for filtering the target materials from water. Coconut
carbon is mainly for filtering airborne materials.
PORE SIZE
When a carbon-based material
is considered for conversion to activated carbon,
test runs are performed to get an idea as to the internal
pore structure of the material after activation. The
final performance of a carbon is based upon the types
and number of internal pore sizes, the internal surface
area and percent of ash in the carbon. The most important
determining factor is pore structure. In the case
of coconut-carbon, due to the coconut shell structure
when it is activated, coconut based carbons have only
very small pores when created. These are called micropores
which are classified as pores with a size less than
40 angstroms. The problem is the DOC in the water
that the carbon is to remove are much bigger than
40 angstroms. So even though coconut carbon has a
large internal surface area, the pollutant can't come
in contact with most of it because it does not fit
inside the pores. Therefore, the activity of coconut
carbon is relatively poor in water.
Carbon made from peat is at the
opposite extreme compared to coconut carbons. When
processed into activated carbon, peat forms pores
that are large, above 5,000 angstroms, and called
macropores. The large pore size permits the water
pollutants to enter the carbon particle where they
are adsorbed. Thus, peat is a better GAC than coconut
based materials for water use. However, the large
pores cause a large reduction in the internal surface
of carbon compared to others which means a loss in
adsorption potential. Thus, while peat does work in
water, it is not the best carbon available.
The best carbon for use in water
filtration to remove the dissolved organics is bituminous
coal-based carbon. Carbon from this base material
contains a large amount of pores that are between
40 and 5,000 angstroms (called the transitional pores).
Bituminous coal based carbons have the right mix of
pore size and internal surface area such that they
remove more pollutants per gram of carbon than any
of the other types of base materials. Generally, carbon
from bituminous coal will be slightly more expensive
than others but they remove far more material. Thus,
they are actually more economical--the carbon of choice
in aquaria is bituminous coal based.
SOURCES OF CARBON
However, as always seems to be
the case, not all bituminous coal based carbons are
the same. Another new development since 1993 is the
importation of activated carbon made in China. Previously,
most of the sources of carbon were in the United States
(domestic carbon) with some imported from Germany
and other parts of Europe. However, nowadays China
is becoming a major supplier of carbon. There are
many sources of coal in China and the prices paid
in the United States for activated carbon make it
economical for Chinese activated carbon to be shipped
here. Chinese carbons vary widely in quality; what
makes it attractive to many companies is that it can
be much cheaper than domestic carbon. Unfortunately,
in this case cheaper generally means inferior in quality
so that the end-user is required to use twice as much,
or more, to accomplish the filtration of one unit
of domestic carbon. Thus, the Chinese carbon is not
really cheaper to use. Few, if any companies, label
the source of their carbon, so it can be hard to tell
domestic bituminous coal based carbons from Chinese
ones. Most of the larger aquarium product companies
sell domestically produced activated carbon. However,
a guide is that the Chinese carbons tend to be much
duster than domestic carbons and more shiny. This
is not 100% accurate, but it is a tendency.
CARBON SUBSTITUTES
The use of carbon substitutes
has increased since 1993. There are a few products
available which remove some of the same compounds
(namely DOC) that activated carbon does. The effective
ones are ion exchange resins, the ineffective ones
are those that look like pebbles or pea gravel. The
ion exchange resin substitutes can be good, but they
clog rapidly if you don't use good mechanical filtration
and they are expensive. Also due to their small size,
they can pack tightly together forcing the water to
go around them instead of through them. If you use
these make sure to stir them up once a week so they
remain effective.
CARBON AND REEF AQUARIA
Another change from 1993 is the
use of GAC in reef tanks. In the earlier years of
reefkeeping many of the "gurus" said, and
wrote, that carbon should not be used in reef tanks
for a number of reasons. Today, most reefkeepers,
even the well-known ones, use some form of GAC in
their tanks--some even sell their own mix of carbons.
What was the thinking behind the concern of using
GAC in reef tanks? The answer is trace elements. It
is repeatedly stated that activated carbon removes
trace elements from water. And, in fact, the statement
is true but it is not complete. It is half of a statement
and therein lies the problem. The complete sentence
should be: activated carbon can remove some trace
elements from water depending upon their concentration
and, most importantly, the pH of the water. I'll explain.
TRACE ELEMENTS
First, what are some the trace
elements in seawater? By most definitions trace elements
are those elements found in the ocean with a concentration
of less than 1 ppb. These are elements such as cesium,
chromium, cadmium, selenium, cobalt, silver, lead,
tin, helium, lanthanum and cerium. It has never been
shown that most of these elements are really needed
in the aquarium. In fact adding some to the water
would really be poisoning the tank. However, in the
aquarium hobby most lists of trace elements contain
copper, iodine, iron and molybdenum. These really
called minor elements.
In any case, whether a minor
or trace element, most can have different chemical
forms in water and not every form can be adsorbed
by carbon (in fact most cannot). The form depends
upon the pH of the water. It turns out that for most
elements, the chemical form that carbon can remove
is only present when the water pH is either below
3 or above 10. These are hardly pH conditions found
in an aquarium. Thus, carbon as used in the aquarium
environment will not remove most of the trace elements
present in sea salt mixes or additives.
PHOSPHOROUS
Phosphorous is another element
linked to activated carbon. Some carbons leach phosphate
into the aquarium water. The phosphate can be a naturally
occurring part of the carbon or it can be from phosphoric
acid which is used in the activation process. In either
case, the phosphate is not toxic, rather it can contribute
to eutrophic conditions in the aquarium water and
may lead to algae blooms. If you are concerned about
phosphates, switch brands of carbon. There are several
bituminous coal-based carbons available from reputable
companies that are phosphate-free.
DE-ADSORPTION
De-adsorption is another phenomenon
that is over-stated in the rumor mills about activated
carbon. Again, it is an incomplete statement that
is commonly used to described the process. It goes,
in one fashion or another, as: don't use carbon because
once its adsorption sites are full it will release,
or de-adsorb, all the stuff it has adsorbed releasing
a large amount of pollutants back into the aquarium.
The implication in this sentence that activated carbon
works something like a capacitor such that once at
its maximum adsorption capacity, it instantaneously
discharges all the bad things it has adsorbed is wrong.
Carbon does de-adsorb, in fact, that ability is exploited
for recycling precious metals. However, in a controlled
industrial process, the quick release of the target
substance is accomplished by switching the pH of the
water. The basic process is to capture the target
substance at one pH extreme (very acidic or basic)
and then reclaim the substance by switching to the
other pH extreme. As stated earlier in this article,
these pH values are outside the normal range of aquaria.
De-adsorption is not a process to be worried about.
GAC AND DISEASES
Carbon has also been implicated
as the cause of Hole-in-the-head disease and lateral-line-erosion.
One theory has the carbon breaking into microscopic
particles which get lodged under the scales of the
fish and infects them with a virus or bacteria. There
have been no published studying confirming this or
any other disease caused by GAC. Further, there are
many, many fish in tanks with carbon that don't suffer
from these maladies. From personal experience, with
both freshwater and saltwater fish, the cause of these
diseases seems to be related to diet, as fish with
these problems can be ''cured" by the proper
diet.
CARBON AND pH
Sometimes putting fresh carbon
in your aquarium can raise the pH as the carbon can
adsorb carbon dioxide and change other characteristics
depending upon the water chemistry of the aquarium
water and the type of carbon. Usually the pH change
is temporary, but how long depends upon the alkalinity
of your water. To minimize this, it is best to rinse
the carbon under tap water for a minute before placing
it in your tank. This will reduce the magnitude of
any changes.
USING GAC
Activated carbon should be part
of every aquarium filtration system. The removal of
dissolved organic compounds from the water will increase
the effectiveness of the biological filter, make the
water look cleaner because it will be clearer, and
eliminate smells. For most filters, GAC comes conveniently
pre-packaged so that it can be easily removed from
the filter and replaced. The most efficient method
is to get as much of the water to go through the carbon
on each pass through the filter. This can be done
by spreading the carbon out in a thin layer rather
than letting it pile up in a bag.
Common usage questions about
carbon are: how much carbon do I need in my tank,
how often should I change it, and how come power filters
don't have much carbon? Only through trial and error
can you accurately determine how much carbon your
tank/filter needs and how often you should change
it. Every tank is different so guidelines are, at
best, guesses. However, for most hobbyists the factor
which will determine how much carbon they can use
at one time is their filter. The most common filter,
the power filter, does not contain a lot of carbon
in their chemical filtration section compared to canister
filters for a necessary reason. These types of filters
work by gravity. Water is pumped up above the tank
water line into a box which hangs on the back of the
aquarium and flows over a weir in the box back into
the tank. The carbon is placed in the flow path of
the water using a cartridge or similar device. The
more carbon in the cartridge, the greater the resistance
to the water flowing back into the aquaria. If too
much carbon is used, the water will overflow the top
of the filter box and spill onto your floor. The overriding
design criteria for today's power filters is convenience.
The older style power filters which used a siphon
to get water into the filter box could hold a lot
more carbon, but it was not easy to change. The best
advice for power filter owners is to change the carbon
once a month and clean the filter pad every few weeks.
For most aquarists, if your tank
is not overcrowded or overfed, then changing the carbon
once a month is probably also sufficient. If you have
big fish or a lot of fish, then you might have to
change the carbon every 2 or 3 weeks. Only by keeping
an aquarium logbook and noting when you change the
carbon and the conditions of the tank before the change
(water color, smells, etc.) can you accurately predict
the correct amount of carbon required for your situation.
To keep your carbon working longer,
make sure to mechanically filter the water before
it reaches the carbon as this will keep the carbon
from clogging with debris. A good practice is to rinse
the carbon under water for a few seconds each week
to get rid of particulate material. This will keep
the water paths open and help you get the full benefit
of the carbon.
Lastly, carbon is not a substitute
for water changes--nothing is. Carbon is a part of
a filtration system and maintenance regime that when
used regularly, will result in less work for you and
healthier, longer-lived fish. □
© 1998 Dr. Timothy A. Hovanec