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Subject: [FAQ] Aquaria: Filters
This article was archived around: Thu, 13 Jan 2000 08:04:54 GMT
* This is only a text dump of part of the Aquaria FAQs. *
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* and contains enhanced content not available in this posted version! *
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FAQ: Aquarium Filtration
contributed by Bruce Hallman
This article describes how filtration can help ensure a healthy
aquarium. The first half describes what filters are, and how they
work. The second half evaluates the different types of filters.
The FAQs owe their existence to the contributors of the net, and as
such it belongs to the readers of rec.aquaria and alt.aquaria.
Articles with attributions are copyrighted by their original authors.
Copies of the FAQs can be made freely, as long as it is distributed at
no charge, and the disclaimers and the copyright notice are included.
Table of Contents
1. Why you need filtration and how it works
+ Water changes
+ Biological filtration
+ Mechanical filtration
+ Chemical filtration
2. Filter types
+ Power filter
+ Canister and Submersible
+ Wet/Dry (Trickle)
+ Protein skimmers
+ Fluidized bed
+ Algal Scrubbers
3. For More Information
1. Why you need Filtration
Sometimes we forget that fish kept in an aquarium are confined to a
very small quantity of water as compared to their natural habitats in
the wild. In the wild, fish wastes are instantly diluted. But in an
aquarium, waste products can quickly build up to toxic levels.
These waste products include ammonia released from your fishes' gills,
fish poop, and scraps of uneaten food. The food and the poop will also
eventually decay, releasing ammonia. Even small amounts of ammonia
will kill your fish.
Obviously, the more sources of fish waste, the quicker and greater the
ammonia problem. A small heavily-fed tank with lots of large fish will
have much more ammonia than a large tank with one seldom-fed small
fish. But for both these cases you need some form of aquarium
filtration to control the toxic ammonia.
Some aquarists try to control ammonia levels exclusively by changing
the water. This is helpful, but impractical because of the frequency
and size of the water changes required.
Fortunately, there is an easier way! In fact, the world is full of
bacteria that want nothing more than to consume the ammonia and
convert it into less toxic substances. For many an aquarist, this
process occurs without their knowledge or help. However, the smart
aquarist will learn how to take advantage of this beneficial bacteria
by maximizing its growth.
When you start a new fish tank, colonies of beneficial bacteria have
not yet had the chance to grow. For a period of several weeks this is
hazardous to fish. You must gradually build up the source of ammonia
(i.e., start with only one or two small fish) to allow time for the
beneficial bacteria to grow. This is called ``cycling'' your tank.
Read more about this in the BEGINNER FAQ.
Remember that the bacteria break down the ammonia into substances
(first nitrite, then eventually nitrate) that are merely less toxic,
rather than non-toxic. Many fish can tolerate reasonably high levels
of nitrates, but over time the nitrates will accumulate until they,
too, become toxic. Also, because nitrate is a fertilizer, high nitrate
levels can lead to excess algae growth.
Although there are many ways to remove excess nitrate, the most
effective way is to regularly change part of the water. This is one of
the most neglected and important parts of aquarium maintenance!
How often and how much you need to change depends a lot on the waste
load in your tank, and the sensitivity of your fish. You don't want to
change ALL of the water at any point in time because the change in
water chemistry will be stressful to your fish. The best way to decide
how often and how much to change your water is to monitor your water
quality with water tests. As a minimum, if your tank is new, you
should test for ammonia and perhaps nitrite. In established tanks you
should monitor for nitrate accumulation. Read more about water tests
in the TEST KIT SECTION of the BEGINNER FAQ. Water tests are the most
reliable way to know how well your aquarium filtration works.
For an average tank, you should change no more than one third of the
water in 24 hours. Many aquarists with average aquariums change a
quarter of the water every two weeks. Your aquarium is probably not
average, and you really should measure nitrate levels to determine
your water change schedule.
Biological filtration is the term for fostering ammonia-neutralizing
bacteria growth. It is so important to the health of your aquarium
that we should look at how this process works more closely. (There are
other types of wastes that can cause problems, but the regular partial
water changes needed to control nitrates are typically enough to
control other forms of waste as well.)
Mother Nature provides several types of bacteria that break down
ammonia into progressively less toxic compounds, nitrite and nitrate.
These bacteria are not harmful and are quite abundant in nature. They
are so common that we do not need to add them to our aquariums; nature
does it for us.
In the presence of ammonia and oxygen these bacteria will naturally
multiply. The bacteria attach to the tank, rocks, gravel, and even
tank decorations. Note that we have not yet said anything about a
physical filter. This is because biofiltration bacteria require only
1. A surface upon which to attach,
2. ammonia for food, and
3. oxygen-rich water.
This sounds so simple; why do we need a physical filter?
Actually, if you limit the amount of fish to what the natural
biofiltration can handle, you do not need a physical filter.
Unfortunately, you cannot support very many fish with only the natural
In the last few decades, the hobby has seen many new types of
biological filters invented which can vastly increase the capacity of
the bacteria colony to provide biological filtration to your aquarium.
In essence, all of these types of filters provide additional surface
area for bacteria attachment and increase the available oxygen
dissolved in the water.
Remember that ammonia comes directly from the gills of your fish, but
also from decaying fish poop and food scraps. If you can mechanically
filter out the poop and the waste food before it gets a chance to
decay, you can be a step ahead in the game. Not to mention that these
wastes are ugly and can detract from the beauty and enjoyment of your
Simply stated, mechanical filtration is the straining of solid
particles from the aquarium water. Mechanical filtration does not
directly remove dissolved ammonia. Most common mechanical filter media
do not remove microscopic bacteria and algae from the water. Neither
will mechanical filtration remove any solids trapped by gravel,
plants, or decorations.
You will need another method to remove the solid wastes from the nooks
and crannies of your aquarium. One of the easiest methods is to
``vacuum'' the gravel, etc., as part of your regular water change
routine and everybody should do this. (Note that those marine
aquariums which use ``live substrates'' are an exception.) Some people
install circulation pumps, known as wave makers, to improve the chance
of catching solid wastes in the mechanical filter.
The four most popular mechanical filtration media are sponges, paper
cartridges, loose and bonded floss media which are reusable to
different degrees. Clean paper cartridges have the smallest openings
and clean bonded floss has the largest openings. Clean sponges and
clean loose floss fall somewhere between.
A filter media with small openings catches finer particles, but clogs
faster. Also, as a rule, a physically large filter area will clog more
slowly than a small filter. As the filter media gets dirty it will
trap smaller and smaller particles. At some point the media is so
clogged that it will not pass water.
SUMMARY: A good mechanical filter is one that traps enough solids to
keep the water clear without plugging too often.
Chemical filtration, in short, is the removal of dissolved wastes from
aquarium water. Dissolved wastes exist in the water at a molecular
level, and fall into two general categories, polar and nonpolar. The
most common chemical filtration method involves filtering the water
through gas activated carbon which works best on the nonpolar wastes
(but also removes polar wastes). Another effective method is protein
skimming, which removes polar wastes such as dissolved organics.
Granular activated carbon (GAC) is manufactured from carbon, typically
coal, heated in the presence of steam at very high heat. This process
causes the carbon to develop huge numbers of tiny pores, which trap
nonpolar wastes at the molecular levels by means of adsorption and ion
exchange, and removes heavy metals and organic molecules, which are
the source of undesirable colors and odors, through a process known as
The best GAC for filtering water is made from coal and is macroporous
(having larger pores). A good macroporous activated carbon feels light
(not dense) and fizzes and floats when initially wetted. GAC intended
for removing wastes from air (such as odors) are commonly made from
coconut shell and are microporous. Carbons for filtering air feel more
Some people (especially those with reef aquaria) are concerned about
phosphate leaching from activated carbons. As a rule, buy only carbons
made by reputable aquarium supply companies which have been acid
washed during manufacture to minimize ash content. Carbons low in ash
also help reduce the chance of undesirable pH shifts. Low ash carbons
typically have lower phosphate leaching levels too.
The phosphate in GAC stems from the fact that activated carbon is
manufactured from coal, which was once living plant matter. All living
matter is high in phosphates. The leaching of phosphate from GAC is
known to be high initially and to decrease over time. This problem can
be mitigated significantly by presoaking your activated carbon for a
few weeks before use.
Some people are concerned about GAC removing trace elements required
by plants and invertebrates for healthy growth. Trace element
depletion is a problem in planted aquaria and minireefs, with or
without activated carbon. The potential benefits of activated carbon
are great enough that on whole you will be better off using it. If
trace element depletion is a worry, use a trace element supplement in
conjunction with the activated carbon.
GAC cannot be rejuvenated outside a laboratory, but fortunately, it is
cheap enough to use liberally. Always wash your carbon before use to
remove the dust that accumulates during shipment. Advice on how much
to use vary, but smaller amounts changed more frequently seem to work
best. You probably want to experiment, but 1/2 cup per 20 gallons
water, changed monthly is a good starting point. In summary, activated
carbon is an excellent, cheap and effective filtration method which is
highly recommended for all aquaria.
A variety of special chemical filtration media have been developed to
remove specific chemicals. A common one is made from the zeolite clay
(also used as some cat litters), and is marketed under such brand
names as ``Ammo-Carb''. This media removes ammonia from water, and is
good for short term use. The aquarist should be aware that if zeolite
is used, especially when cycling a new aquarium, then the
establishment of natural biological filtration will be delayed or
Protein skimmers are primarily used in saltwater aquaria, especially
reefs. They have the remarkable ability to remove dissolved organic
wastes before they decompose. The process involves taking advantage of
the polar nature of the organic molecules, which are attracted to the
surface of air bubbles injected into a column of water. The resultant
foam is skimmed off and discarded.
2. FILTER TYPES
The humble corner filter
For decades, hobbyists have successfully kept fish healthy and happy
through the use of the $2.49 corner filter. Typically, they are clear
plastic boxes, which sit inside the tank. An air stone bubbles air
through an air lift tube, which forces water through a bed of filter
floss or other media. mechanically filtering the water. Colonies of
bacteria build up on the media, providing excellent biological
filtration. (It is important to change only some of the media at any
given time! This way the bacteria does not get wiped out.) Nowadays
people don't use corner filters as much because they're ugly, take up
space in the tank, and require a bit more frequent maintenance than
other filters. But you can't beat the price.
Another use of the corner filter, that is not really matched by other
filter types, is as an impromptu quarantine tank filter. If you have
the need to set up a second tank on the quick, you can take some
gravel from an established tank and put it in a corner filter, and
immediately, you will have a functioning biological filter. This way
you can turn a five gallon bucket into a quick and cheap
hospital/quarantine tank on a moment's notice.
Fish stores commonly sell undergravel filters (UGF's) to beginners in
``aquarium kits'' because they are cheap, and they work (for a while).
UGF's work by slowly passing water through the bottom gravel, which
sits on top of a perforated plate. The water can be pumped with an air
lift, with bubbles air lifting the water in a vertical tube attached
to the filter plate. Also, some people prefer the increased water flow
achieved with submersible pumps, called powerheads, attached to the
same lift tubes.
UGF's make good biological filters, because the slow flow of water
through the gravel fosters large colonies of beneficial bacteria which
neutralize toxic ammonia. The hitch is, that UGF's are awful
mechanical filters. Fish waste gets pulled out of sight into the
gravel. Before you know it, the gravel clogs up. You then have a big
mess and a health risk to your fish!
A partial solution to this dilemma is to run the powerhead in reverse,
sending water up through the gravel. This technique is known as
Reverse-flow Undergravel Filtration (RUGF); conversion kits or special
powerheads can be purchased to accomplish this. The intake of the
powerhead is covered with a porous sponge which serves to "prefilter"
out some of the waste that can clog the gravel. In actually practice,
this helps, but is only a partial solution.
If you choose to use an UGF/RUGF, you must regularly vacuum your
gravel. Fish stores sell siphon hoses with a ``wide mouth gravel
vacuum tube'' attachment that ``washes'' the gravel during your
regular water changes. IF you clean your gravel regularly, and
maintain a regular and frequent partial water routine, UGF's and
RUGF's are an economical and effective aquarium filter in freshwater
aquariums, and in lightly stocked saltwater fish-only aquariums.
Sponge filters provide an efficient and cheap form of biological
filtration. Water is forced through a porous foam, either by a
powerhead, or air bubbling through an airlift tube. Water flowing
though the sponge allows the growth of a colony of beneficial bacteria
which neutralizes toxic ammonia.
One style of sponge filter uses two sponges attached to one lift tube.
These have the advantage that the sponges can be cleaned one at a
time, reducing bacterial loss. Also, one of the sponges can be removed
and transferred to a new tank, bringing with it a colony of beneficial
bacteria, and thereby "jump starting" the cycling of a new tank. Some
enlightened fish stores sell these double sponge filters to beginner
customers when they sell a tank kit. They take one of the new sponges
out of the "box" and swap it for a old established sponge in one of
their tanks in their store which is carried home in a plastic bag.
Most people agree that power filters are much easier to maintain and
can be as economical as undergravel filters. There are many styles of
power filters, but the most common hangs on the back of the tank. A
siphon tube pulls water from the tank into the filter box and passes
the water though a mechanical filter (typically a porous foam sponge).
The sponge doubles as a biological filter. A internal pump then
returns the filtered water into the aquarium. These power filters come
in many sizes suited for small to large aquariums.
The foam sponge can be easily inspected for clogging or removed for
cleaning. You must clean the sponge regularly to remove the solid
wastes before they decompose and dissolve back into the water. It is
quite important that when you clean the porous foam that you do not
kill the bacteria colony through the use of detergents, very hot or
very cold water. A safe and easy way is to rinse the foam sponge in
the bucket into which you have just drained some tank water during
your regular water change routine.
Power filters now come with all sorts of fancy ``features''. Most
allow placement of a chemical filtering media, typically granular
activated carbon, in the water path.
Another development in the last few years is the ``wet-dry wheel''
(called a biowheel by one manufacturer). The beneficial bacterial
colonies that neutralize toxic ammonia require an oxygen rich
environment to grow. The ``wet-dry wheel'' passes water over a water
wheel device which sits outside (on the edge) of the aquarium. This
rotating wheel maximizes available oxygen allowing a large bacteria
colony to flourish. One drawback is that these wheels have been known
to jam, so you need to check them frequently. Other than this minor
point, the ``wet-dry wheel'' is an excellent method of providing
vigorous biological filtration.
The Canister filter
Canister filters have some similarities with the ``hang on tank''
style of power filters, but the essential difference is that canister
filters are designed to provide more powerful mechanical filtration.
Typically, the water is pumped, at moderate pressure through a filter
material, such as glass wool, or a micron filter cartridge. Canister
filters are especially useful in aquaria with large or numerous messy
eaters that generate a lot of waste. For these filters to be effective
they must be frequently cleaned, to avoid the decomposition of waste
in the water stream.
These filters usually sit on the floor below the tank, but also can
hang on the tank, and in some designs even sit inside the tank, in
which case they are called a ``submersible filter''. Some hobbyists
attach a ``wet-dry wheel'' to the outflow of their canister to improve
the biological filtration capacity of this type of filtration system.
Also known as trickle filters, wet/dry filters work on the principle
that the beneficial colonies of ammonia neutralizing bacteria grow
best in the presence of well oxygenated water. By ``trickling'' water
over unsubmerged plastic gizmos or other media, wet/dry filters
provide a very large air/water surface area. They come in many shapes
and sizes. The boom in successful saltwater aquariums in the 1980's
can be attributed to the use of this filter type.
Many things can used for the media, with the best providing great
amounts of surface area, while at the same time having large openings
to reduce the tendency to clog and ensure efficient gas exchange. The
problem of clogging of the media can also be reduced by prefiltering
the water with an efficient mechanical filter, and (when used) with a
Protein skimmers (aka Foam Fractionators)
Protein skimmers were initially developed for use in industrial sewage
treatment plants where they are also known by the term foam
fractionator. Protein skimmers have the unique ability to remove
dissolved organic wastes BEFORE they decompose! This is a neat trick
which is accomplished by taking advantage of the fact that organic
chemicals are attracted to the surfaces of bubbles which are passed in
large numbers through a column of water. The foam is then ``skimmed''
off the water, while at the same time removing the organic wastes. The
foaming process only works in a water with high pH and salinity, and
as a result skimmers are primarily for saltwater use.
The protein skimmer is largely responsible for the boom in reef
aquaria in the 1990's, due to the high water quality possible with
this type of filtration. A current ``state of the art'' in reef
systems is based upon the use of protein skimmers and live rock
without the use of a wet/dry filter. This school of thought is known
as the ``Berlin method''.
Fluidized bed filters
Very recently, some hobbyists have reported success with a new type of
filter which uses a fluidized bed of sand. This filter is roughly
similar in principle to the reverse flow undergravel filter, but with
much higher water flow. The higher water flow keeps the sand clean of
debris, while at the same time allowing the development of large and
efficient colonies of beneficial bacteria. Reported problems include
oxygen depletion and clogging.
Another specialized type of filter is designed to help in the control
of the accumulation of nitrates, the end product of the neutralization
of ammonia by the biological activity of bacteria. These fall into two
categories, the anoxic bacterial, and the plant/algal scrubbers
(discussed in the next section). It has been discovered that colonies
of bacteria which grow in oxygen poor environments can be harnessed to
biologically consume nitrate, and release harmless nitrogen gas. This
method is achieved in one of two ways. The process was first developed
in the 1980's through the use of a box system, coil, or porous foam
block which allowed very slow transmission of nitrate-laden water.
Inside the box/coil/foam, sugar was placed, and the slow passage of
water quickly became anoxic. In these anoxic conditions, bacteria
would grow and consume excess nitrate. Many aquarists have reported
failure in their attempts at this type of filtration.
More recently, hobbyists have developed similar anoxic conditions
below plates at the bottom of their tanks buried in fine sand. In the
saltwater systems, these sand beds are referred to as "live sand". In
freshwater planted systems, fine grain substrates are allowed to
develop anoxic zones which probably also have a denitrification
The Berlin Method of reef aquariums involves the use of large
quantities of live rock harvested from tropical reefs. Aquarists
report good nitrate control in live rock systems, which, though not
well understood, probably involves the denitrification of the nitrates
within the interior of the rocks. Another school of thought is that
the heavy growths of calcareous algae on the live rocks in Berlin
Method reef aquariums consume nitrate.
Algal scrubbers use live algae to do the ``filtration''. Water is run
over a wire mesh in a trough under bright lights, where algae is
encouraged to grow. The growth of the algae removes some pollutants
from the water. This is a controversial form of filtration for reefs
and large marine ecosystems invented by Dr. Adey at the Smithsonian.
Some believe it is a complete filtration solution, others claim its
use leads to poor water quality and algae growth in the tank as well.
In freshwater planted aquariums vigorous plant growth has been
observed to beneficially consume excess dissolved nitrates.
While not really a filtration, saltwater aquarists occasionally have
the need to lower the temperature of their aquarium water. The high
light levels needed in reef aquaria involve a build up of excess heat.
Use of a hood fan and removal of the ballast from the vicinity of the
tank can also help. Submerged pumps are also a source of unwanted
heat, and as a solution, reef aquarists favor the ``non-submerged''
pumps due to the decreased transfer of heat to the water.
A little recognized source of heat control is through the natural
cooling effect of evaporation in wet dry filters, and through the flow
of air over the surface of the aquarium. Nevertheless, additional
cooling is often required, especially in warm climates.
This is achieved through the use of "freon" style cooler units similar
to home refrigerators. They either pass the water through a heat
exchange unit, or pass coolant through a heat exchanger in the tank.
Those chillers are expensive but not many people have had success in
the "do it yourself" construction of chillers. (The "dorm" type of
refrigerator is not powerful enough to be of use, just in case you
were thinking about this.)
In especially sensitive aquaria, infections resulting from water born
parasites, fungi, bacterium and vires can cause serious problems.
Water sterilization is most beneficial for breeders (as it can help
control infections of incubating eggs), for centralized multi-tank
filtration (to control the spread of disease between tanks), and for
delicate and/or costly setups such as large tanks and reef systems (as
a safety measure). It is important to remember that a healthy aquarium
depends on beneficial bacteria typically growing on media in your
filter which neutralize ammonia. At most, your sterilizer can kill
some water born pathogens, but total sterilization is not possible or
desirable. Aquarists who practice prudent quarantine procedures for
newly acquired fish generally do not need to sterilize.
Two main types of sterilization are used, ozone injection and
Ozone gas is highly reactive and is a powerful oxidizer of organic
pollutants, including living pathogens. Another benefit of water
treatment with ozone gas is that it systematically reduces dissolved
organic compounds in the water stream which increases the reserve
capacity of the water to oxidize organic waste throughout the
aquarium. Ozone laden water also improves the ability of protein
skimmers to generate foam which increases their overall performance.
Prior to the discovery of the live rock/protein skimmer "Berlin
Method" style of reef keeping, ozone injection was considered part of
a "state of the art" filtration system, especially among Europeans in
the 1980's. The trend of late is towards the more simple and natural
Berlin Method. Though ozone use remains beneficial, it is being used
less in recent years among reef keepers.
Ozone gas is produced by devices which create a spark in dry air. As
humidity drastically reduces the efficiency of ozone generators most
aquarist choose to pretreat the air for the ozonizer with a
dehumidifier. Ozone gas is highly corrosive, all elements (especially
rubber) which can come in contact with ozone must be made from ozone
safe materials (commonly silicone). Residual ozone can be efficiently
stripped from air by passing the air through activated carbon. Ozone
must not be allowed to enter your aquarium because it can kill your
fish and invertebrates and/or damage the beneficial bacterial in your
biological filter. Also, ozone gas is unsafe to breath and can cause
irritation even in small concentrations.
High intensity ultraviolet light destroys the DNA in living cells and
can be an effective means to control living pathogens. The most
effective UV light is the high energy UV(C) light roughly at the
wavelength of 250 Angstroms. To be effective, UV Sterilization (UVS)
must expose the pathogens to high enough light intensity for a long
enough period of time. Martin Moe cites 35,000 to 100,000 microwatts
per second per square centimeter as the norm, which works out to
roughly 10 to 25 gallons per hour per watt (or less for units not
operating at peak efficiency).
Common problems which can reduce efficiency and kill rate are:
1. Allowing the water to flow too fast past the UV light.
2. Light blockage due to a build up of salt deposits or bacterial
slime on the bulb.
3. Fading of the light due to age of the bulb (which typically have a
six month life.)
The same property of this light that kills germs can damage your eyes,
and special care MUST BE TAKEN to avoid direct or indirect eye contact
with this light. [This is especially serious because the damage occurs
inside your eyes before you feel any pain. Too many people have
already damaged their eyes in this way!] The UV(C) light does not
penetrate water very well, so to be effective, UV Sterilizers commonly
position the UV bulb close to the water which also can pose a risk of
electrical shock should the bulb break, etc..
There are three types of UV Sterilizers:
1. Tray type. (Typically homemade) with UV bulbs suspended in a
reflecting fixture over a shallow tray of slow flowing water.
Benefits: easily cleaned, can be cheap, can be made large enough
for commercial applications. Problems: safety risks to your eyes,
too large and awkward for many home uses.
2. Tube type, wet bulb. Tube types have the benefit of exposing all
sides of the UV tube to water with no reflector. The water passes
directly past the bulb which is mounted in a waterproof tube.
Benefits: cheap, compact and effective. Problems: difficult to
clean the slime accumulations from the bulb, safety risks due to
3. Tube type, dry bulb. Similar to above, but the UV tube is
surrounded by a quartz tube [glass blocks UV(C) light] insulating
it from the water. These are more expensive and probably safer.
Changing the light bulb is easier and dry bulb tube types can have
a internal device to wipe slime from the quartz tube. Some of
these types come with sensors to monitor the intensity of the
light to let you know when to replace/clean the bulb. etc..
To learn more
See the RESOURCE FAQ for several good books. A good reference work for
aquarium filtration is Marine Aquarium Reference (Systems and
Invertebrates) by Martin Moe