Aquarium Filtration
Summary
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.
Copyright
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such it belongs to the readers of rec.aquaria and alt.aquaria.
Articles with attributions are copyrighted by their original authors.
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Table of Contents
- Why you need filtration and how it works
- Introduction
- Water changes
- Biological filtration
- Mechanical filtration
- Chemical filtration
- Filter types
- For More Information
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.
Water changes
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
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
- A surface upon which to attach,
- ammonia for food, and
- 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 biofiltration.
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.
Mechanical filtration
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 aquarium.
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
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 molecular sieving.
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 dense.
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
disrupted.
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.
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.
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 skimmer.
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''.
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 capability.
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 ultraviolet
irradiation.:
Ozone
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.
Ultraviolet Sterilizers
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:
- Allowing the water to flow too fast past the UV light.
- Light blockage due to a build up of salt deposits or
bacterial slime on the bulb.
- 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:
- 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.
- 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 electrical shock.
- 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..
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
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