The following
is a series of questions asked of Leo Morin, Ph.D., by Bruce
Hallman on December 28, 1995.
 BH>
Is the use of activated carbon in aquarium filtration worth
the risk?
LM> I think the benefits far outweigh the risks, particularly
in non-reef aquaria. Even in a reef aquarium, it is possible
to use carbon and minimize the risk by selecting the right carbon
and pre-soaking it in DI water for several days to leach out
the majority of the leachable phosphate.
BH> Do we need to worry about buying activated carbon which
has been washed with phosphoric acid?
LM> That question is based on the common misconception that
phosphate in carbon arises from acid washing with phosphoric
acid. I know of no major carbon today that is acid-washed in
phosphoric acid. Acid-washed carbons are preferable because
they contain less, not more, leachables such as phosphate. Acid
washing is usually done with hydrochloric or sulfuric acids,
not phosphoric acid. The phosphate in carbon arises from the
organic or "once living" source of all carbons. All
such materials will be rich in phosphate because all living
matter is rich in phosphate. DNA, RNA, energy transfer molecules,
and a host of other important biological compounds are phosphates.
BH> Do the "premium" activated carbons leach less
phosphate as a rule than the "budget" activated carbons?
LM> That is not a blanket rule. Acid-washed carbons leach
less phosphate than others and these are usually more expensive.
Coconut carbon leaches less than coal based carbons, but that
is because coconut carbon is microporous and has a slow rate
of adsorbtion as well as leaching in water, since it is enginered
mainly for gas filtration rather than water filtration.
BH> Should I buy "brand name" activated carbon,
or is "generic/bulk" OK?
LM> Again, that is hard to answer. What is important is identifying
a good carbon. Brand or generic doesn't really matter.
BH> How much should I use? How often should I change it?
LM>I prefer to use little and change it frequently rather
than more and changing it infrequently. I recommend around 100
mL for each 20-40 gallons and changing it at least once a month
or sooner.
BH> An "expert" told me that I should use coconut
shell carbon in my filter because that's the best. Is that good
advice?
LM> That "expert" wasn't very expert. Coconut carbons
are microporous and are excellent for gas filtration, but do
poorly with water filtration. Water filtration, particularly
aquarium filtration, requires a macroporous carbon. The easiest
way to judge that is the carbon density. The lighter weight
the carbon is for a given volume the better. You will find that
coconut carbons are comparatively dense.
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BH>
But how can I practically choose a "good" activated
carbon? A package of carbon costs typically less than $10, but
a phosphate test kit costs more than $20!
LM> I agree that personal testing of carbons can get a bit
expensive and time consuming.
Phosphate leaching is one parameter for carbon evaluation, but
is not the most important. Since most aquarium carbon suppliers
do not give specifications for their carbon, selection usually
requires some detective work. Our (Seachem) carbon gives all
important specifications on the label. I would suggest the following
guidelines:
1) If the labels gives key specifications (porosity, density,
ash and phosphate content) then the supplier likely has nothing
to hide and the carbon is likely a good one.
2) If the carbon boasts no phosphate, then the supplier is either
lying or doesn't know any better. Neither is very reassuring.
3) Compare weight and volume. The less weight for a given volume,
the greater the porosity and the better the carbon, all else
being equal. You can usually do this without buying the product
first. After buying the product, this parameter will be reflected
by the carbon's ability to float and fizz.
4) In terms of porosity suitable for aquarium filtration, coal
based carbons are best, followed by wood based. Coconut or other
nut shell based carbons are least suitable. This information
may be availble on the label.
5) In terms of ash and phosphate content, acid washed carbons
are better than non-washed carbons. Acid washed carbons have
had much of their ash and phosphate washed out. Ash is important
because it is responsible for "pH shock". Some carbons
can increase pH to over 10 in a very short time. An acid washed
carbon will barely increase pH to 7 over several days. Carbons
that do not alter pH are usually the same carbons that will
not leach much phosphate. Ash content may be available on the
label, but sometimes it is unreliable. The store owner may be
familiar with what the carbon does to pH.
6) Soaking in DI water and testing for phosphate is impractical,
but may be necessary if phopshate is considered an important
parameter. In my view, it is important only for reef aquaria.
7) Some physical characteristics that should be evident before
buying should be considered. Particle size should be about pin-head.
Powdered carbons offer no real advantage and are difficult to
handle. Large particle sizes become impenetrable by water and
so only the outer 1-2 mm of the particles are adsorbent, making
as little as 20% of the carbon useful. Spherical shape is ideal
hydrodynamically for unimpeded water flow and inability to pack,
and is therefore superior to random granular shapes. To my knowledge,
the only spherical carbon available is sold by Seachem.
Find
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