In my previous columns, I was in the process
of creating a checklist for setting up a reef aquarium.In order to continue, it is important to have an understanding
of aquarium filtration so that the most appropriate filtration
method can be chosen.In
this column, I will define the three types of filtration available
to marine aquarists.
“Reef aquarium filtration.” These three simple words comprise one of the
most controversial subjects among reef aquarists, ranking
second only to lighting discussions.
The reason that this is such a controversial subject
is because there are so many different filtration systems
with which reef hobbyists can be successful.
Many different modes of filtration including
biological, mechanical, and chemical filtration, as well as
various combinations thereof exist, for reef aquaria. While all forms of filtration can help with the success
of a reef aquarium, biological filtration is the most essential
to the life and health of our aquarium inhabitants. In an aquarium, we need to make sure that we have enough
biological filtration available to eliminate the amount of
waste created by the inhabitants and chemical processes that
Filtration in Simple Terms
Biological filtration is a process that
uses living organisms to change one form of waste (ammonia)
into another. The breakdown of one of the more toxic forms,
ammonia, into a less toxic form, such as ions of either nitrite
or nitrate, is critical to the survival of our aquariums.
The following discussion of the filtration
types and components is largely based on the in-depth discussion
in “The Aquarium Reference – Systems and Invertebrates”
by Martin A. Moe, Jr., 1992.
Although toxins may come in many forms,
or can be airborne or introduced by contaminants, ammonia
is perhaps the most important. Toxic ammonia accumulates in
the aquarium from the breakdown of fish waste, decaying food,
or other organic material. Additionally, ammonia is the major
component of most marine animal urine. Ammonia can cause damage
to fish tissue, disrupt the oxygen levels in the bloodstream
and cause stress that can bring on disease or animal death.It
is toxic to most marine life so there must be a way to eliminate
or neutralize it in the aquarium water.This is where the bacteria
that are part of our biological filter come into play.The
process called “cycling” refers to the establishment of bacterial
colonies in the filter bed that convert ammonia to nitrite
and then nitrite to nitrate.After an aquarium has completed
its cycle, the bacteria will have colonized the substrate
(surfaces such as rock, sand and even glass) in the aquarium,
and animal life can then be supported.
The bacteria responsible for the initial
step of the nitrification cycles in our aquariums, consuming
ammonia and converting it into nitrite, consist of various
species from the following genera Nitrosomonas, Nitrosospira, Nitrosococcus and Nitrosolobus and others.These nitrifying bacteria are aerobic, which
means that they need oxygen to survive. In an aquarium with
good water flow, these bacteria will colonize all suitable
aerobic surfaces within which the water comes in contact.
The next group of bacteria involved in
the nitrification cycle is responsible for converting the
previously discussed nitrite breakdown product into nitrate.
This is accomplished by numerous bacterial species from the
Nitrosospira, Nitrocystis, Nitrococcus and others.Nitrites
are also toxic to many forms of aquatic life and can inhibit
oxygen uptake in the bloodstream of fish.Fortunately, these
bacteria keep nitrite at low levels in a fully cycled aquarium.
While nitrate is less harmful to our animals
than ammonia or nitrite, it may cause excessive algae problems
that affect the health of our animals.Algae require nitrate
as a basic nutrient.If nitrate is allowed to accumulate in
our reef aquariums in amounts above 5 ppm, it can affect the
health of the corals that we are keeping by polluting the
water and stimulating algal overgrowth of coral tissues.This
waste product, the end product of the nitrification cycle,
may be removed by such means as water changes, chemical adsorption,
protein skimming, uptake by algae filtration or other organisms
that can utilize nitrate as a dissolved nutrient source.
Since I am discussing bacteria, I will
also mention some of the many forms of denitrifying bacteria
found in our aquariums: Micrococcus, Pseudomonas, Denitrobacillas,
Bacillus and others.Some
of these are anaerobic (requiring little or no oxygen) and
some are aerobic.The bacterial genus, Thiobacillus,
consists of eight species.These particular bacteria utilize
organic sulfur compounds such as sulfide, thiosulfate and
bisulfite in low oxygen areas as a substrate to reduce nitrate
in our aquariums.The bacteria consume the nitrate and convert
it to nitrogen gas, which gets released into the air through
water movement and aeration.Some suitable substrates for these
bacteria are in the low oxygen areas in the center of porous
live rock or in the deeper layers of a sand bed.
briefly summarize this discussion of the Nitrogen cycle, please
refer to the following diagram.
Now that we have these wonderful bacteria, where do we put them?
Nitrifying bacteria require a place to
colonize and multiply in order for them to be efficient in
converting waste (nitrogen compounds) in the aquarium.The
media or substrate for colonization can consist of sand, rock,
or man-made products such as bio-balls or other filter media.Since
the main interest of a reefkeeper is a successful reef aquarium,
using more natural forms of substrate is desirable.Our animals
come from an ocean containing both live rock and live sand,
and using natural forms of filtration will more closely mimic
their original environment.Natural forms of substrate consist
of live rock and live sand that will make up the foundation
of the biological filter. On a side note, coral surfaces are also an excellent substrate for bacterial
Live rock consists of calcium based coral
skeletons that are populated with microscopic bacteria and
marine organisms.Because live rock has a porous texture, there
is a large, aerobic surface area for the microscopic nitrifying
bacteria to colonize.Another
property of live rock is its ability to provide areas where
denitrification may occur.Below the aerobic surfaces of live rock are
areas of very low oxygen that are suitable for beneficial
denitrifying bacteria to colonize.
Live sand consists of sand collected from
the ocean that is populated with microscopic bacteria and
marine microorganisms.Non-living sand can also be populated with bacteria
and organisms that will convert it to live sand.In addition to numerous benefits for an aquarium
to be discussed in a future article, sand is an excellent
substrate for nitrifying bacteria.Typically, the sand surface area covering the bottom
of an aquarium is very large, allowing it to perform as an
effective biological filter.Like live rock, sand also works as an excellent denitrification
filter when a bed of four or more inches of sand is used.Sand is so efficient that denitrification can
even occur on a single grain of sand.
Chemical filtration is the process of using
organic or synthetic compounds to remove impurities from the
aquarium water.Two of the more common forms of chemical filtration
are granular activated carbon and molecular adsorption filters.
Granular Activated Carbon (GAC)
While GAC is normally categorized under
chemical filtration, it also has some mechanical filtration
(see below) properties.Granular activated carbon acts as an organic
is very porous, allowing it to trap physical particles.The process of trapping the waste particles is called
absorption.There are also chemical properties that make
carbon attract certain forms of impurities such as phosphate,
organic acids, proteins, metals such as copper, and antibiotic
compounds contained in the aquarium water.The process of attracting these impurities is called
adsorption.One of the best uses that I have found for carbon
is the removal of organic acids that give the aquarium water
the yellow tint that is often seen by reef hobbyists.
Absorption is a process that works similarly
to a sponge.The process
can be compared to washing your car with a sponge.The car is sprayed with water, the sponge is then run
over the surface, it collects dirt but leaves the water behind.Similarly, in the aquarium, water is forced
through a medium (GAC) that traps waste particles; removing
the medium brings the trapped particles along with it.
Adsorption is defined in Webster’s Dictionary
as the capability of a solid substance (adsorbent) to attract
to its surface molecules of a gas or solution (adsorbate)
with which it is in contact.In our aquarium realm, the definition is the
capability of a solid substance (carbon) to attract to its
surface, molecules of a solution (aquarium water with impurities)
with which it is in contact.
Adsorption may be analogous to the example
of static electricity buildup on a computer monitor.The static electricity buildup on a computer monitor
attracts dust particles from the air similar to how carbon
attracts impurities from our aquarium water.The aquarium water (air) flows through the carbon (static
charged computer screen), which attracts the chemical impurities
(dust), binds them, and keeps them from going back into the
absorption, which physically
traps particulate matter, the process of adsorption chemically
attracts impurities and binds them through chemical processes.
An interesting property of GAC is that
it can be produced to remove specific chemical forms of waste
or contaminants from the aquarium water.By producing GAC at different temperatures, sizes,
and textures, or by adding certain chemical elements, it can
selectively remove impurities.
GAC may be used in different ways:It can be placed in a canister filter so that
all water drawn through the filter passes through it for maximum
contact time.It can
also be used passively by being placed behind rockwork or
in a sump where water flows around and through it.While forcing water through GAC is thought to be the
most efficient usage of absorption and adsorption, it will
still work well if used passively, primarily due to its adsorption
Molecular Adsorption Filters
Molecular adsorption filters such as a
PolyFilter™ from Poly-Bio-Marine work similarly to carbon
by using chemical bonding to attract impurities or excess
nutrients from aquarium water.Depending upon their composition, they can be
made specifically to remove toxins such as copper and other
metals, or even excess nutrients such as phosphate.This type of filtration is normally used for short
periods of time, and is best suited for quick removal of excess
organic waste or toxins that may have been accidentally introduced
into the water.While
they can be used for longer periods of time, it is better
to find the source of the problem rather than relying on an
absorption filter to remove chronically problematic levels
of certain substances.
The primary function of mechanical filtration
is to remove large particulate matter from the water before
it begins to decompose.Filter sponges, polyester floss, and
micron filters are some of the more common forms of mechanical
filtration media.While mechanical filtration can be used on
a reef aquarium, it may become a detriment if not used properly.If
the mechanical filter media is not cleaned often, the particulate
matter trapped by the filter will decompose and pollute the
aquarium water. There are times, however, when a mechanical
filter can be used to quickly remove large particulate matter
from the water.Removing unsettled sand from a new aquarium
or when a large amount of particulate matter is introduced
(e.g. disturbing a sand bed or live rock), are events when
mechanical filters can be beneficial.
Protein skimming is one of the more popular
and effective forms of filtration.An advantage of protein
skimming is the ability to remove organics and waste material
from the aquarium before they begin to be broken down by the
The process of skimming involves the following:
||Water and large
amounts of air are injected in to a columnar shaped device.
||The air and saltwater
are forcefully mixed together, creating bubbles.
||The bubbles become
coated with organics and proteins (adsorption), creating
foam (or skimmate.)
||The foam rises up
the column into a collection cup, located outside of the
aquarium, where the organics and proteins are removed
from the water.
Another advantage to protein
skimming in the reef aquarium is that the mixture of air into
the saltwater increases the amount of oxygen in the water,
which, in turn, is beneficial to the health of the aquarium
An in depth explanation of protein skimming
is available in a Reefkeeping Online feature article by Frank
Marini located here.
Summing up, I have explained the three
basic types of aquarium filtration: biological filtration
is required to maintain life in our aquarium; chemical filtration
can be a useful aid in removing impurities from our aquarium;
and mechanical filtration such as protein skimming can help
in continuous waste removal while other forms of mechanical
filtration can aid in the removal of large particulate matter
under special circumstances. Additionally, there are others
forms of filtration that I will discuss in a future article.
you have any questions about this article, please visit my
forum on Reef Central.