Filtering Aquarium Water
For aquatic animals, good water quality is synonymous with good health.
Good water quality is dependent on efficient filtration of the water
in your tank. Understanding filtration is one of the most difficult
tasks facing the new aquariust.
Even the novice aquarist is aware that high levels of ammonia
will quickly spell the end to her fish. Unfortunately, many water
quality problems are more subtle. There is a domino effect set in
motion with deteriorating water quality, ending in the death of a
tank full of pets.
As water quality slowly worsens, the fish suffer from physical
stress. Stress, over time, will cause the health of the fish to
decline. Chronic stress makes fish vulnerable to bacterial and
fungal infections and parasitic infestations. These infections
and infestations will be but the conclusion to the phenomenon
that began with poor water quality caused by inadequate filtration.
An appropriate filtration system and good aquarium management
can minimize, if not completely eliminate, chronic stress. To
better illustrate the correlation between fish health and filtration,
a discussion of the three basic types of pollutants that accumulate in
aquarium water and the corresponding filter options for each kind
There are a number of sources for the particulate matter that
accumulates in aquariums: uneaten fish food, digested waste products,
and cell material from fish and plants. Some of this material remains
in suspension rather than sinking to the bottom, making the water
turbid - cloudy and dirty.
Larger particles sink to the bottom, only to rise again and cloud
the water when disturbed by fish or aquarium maintenance activities.
Besides being aesthetically unappealing, fish and plants are
adversely affected buy turbid water. Turbidity-induced stress
weakens the ability of fish to resist the disease organisms that are
a normal part of the aquatic environment. Gill filaments may become
irritated and begin to swell interfering with respiration,
which heightens stress, and eventually causes gill membranes to become
susceptible to bacterial and parasitic invasion. Deposits of
particulates on plant leaves reduce light adsorption and
gas exchange, and many plants will die as a result.
Finally, suspended particulates may also support blooms of
Daily water changes of about one-fourth of the tanks water and
and in-tank gravel cleaning, would reduce particulate pollution.
Who has time for that? (Not to mention, too much new water can be
quite stressful to some fish.
The best way to deal with suspended material in the water is
with mechanical filtration. This works by collecting particles
from the water as it continuously circulates through a mechanical
screening medium. Mechanical filtering mediums include nylon or
polyester floss, foam sponges, fiber pads, aquarium sand and
These media differ primarily in terms of the size particles they
can effectively capture, their cost, service life, and reusability.
At one extreme is floss, which tends to trap only the largest particles
and cannot be reused, but is inexpensive.
At the other extreme is diatomaceous earth, which can trap extremely
fine particles but is expensive and requires the most maintenance.
The size of the spaces - or pores - in the media,
the shape of the spaces, and the roughness of the material determine
the particulate-trapping ability of mechanical filtering media.
Filtering medium that traps very fine particles will quickly clog
and requires frequent replacement. Otherwise, the water flow will be
restricted to the point where turbidity in the tank actually increases!
A better method is to use filter material that traps larger particles,
such as fiber pads. Over time, the accumulation of larger particles
fills in the spaces, and smaller particles are also trapped.
When the water flow of the filter is significantly reduced,
the filter media is then cleaned or replaced.
An important factor in choosing the mechanical filter is the
rate at which the water will flow through the material. An outside
power filter should circulate, in gallons per hour, four to six times
the volume of the aquarium in gallons. Thus, for a 50-gallon tank,
the filter should process at least 200 gallons per hour.
When choosing from among several filters of the same flow rate,
note which design offers the largest filtering-medium surface area.
A variety of substances such as proteins, amino acids, and
other metabolic byproducts are continuously being discarded into the
aquarium water by the fish. These organic substances are
dissolved into the water - thus the name, dissolved organic carbon (DOCs).
If these DOCs are allowed to become concentrated in the water,
the health of the fish will suffer.
The practical way to deal with DOCs is by chemical filtration
techniques that are reliable and economical. The most common and
reliable are carbon filtration and protein skimmers (foam fractionation).
Carbon filtering removes DOCs via adsorption. As the aquarium
water flows through the carbon medium, the DOCs come into contact
with the surfaces of the carbon granules and become attached.
Some DOC compounds are adsorbed directly, whereas others combine
chemically with already-captured substances.
When the surfaces of the carbon become saturated, it must be
discarded and replaced with new carbon.
Many types of carbon are sold for aquarium use, but only a few are
actually capable of adsorption in water. The material of choice is
high quality granular activated carbon (GAC).
This is carbon that has been degassed in an oxygen oven at
temperatures close to 2,000 degrees Fahrenheit.
The degassing enlarges the apparent surface area of each carbon granule,
greatly increasing the adsorptive capacity.
In a filter, the GAC (carbon) should always be placed after the
mechanical filtering medium. Otherwise, the particulates in the water
will quickly coat the surfaces of the GAC, rendering it useless.
Deep beds of GAC are always more effective than shallow ones of
equal area. Using the smallest granules available maximizes adsorption.
Do not use powdered activated carbon. It compacts too easily and
seriously restricts water flow except in specially designed systems.
There should be 4 to 10 grams of GAC for every gallon of water, and
it should be replaced monthly. Choose the best quality GAC you can find.
Any chamber that houses less that 4 grams per gallon of water is useless
and will only work to restrict water flow.
The most common use of a protein skimmer is in saltwater aquariums,
but they work just as well in freshwater tanks. The only difference
is that the freshwater aquarium must be set up and running for a
while before a protein skimmer works as designed.
GAC beds and protein skimmers remove such substances as hydrogen sulfide,
and other toxic chemicals that may find their way into the aquarium.
Unfortunately these devices are incapable of telling the
bad guys from the good guys and will also remove beneficial
chemicals, such as many medications.
Therefore, it is imperative to remove all
GAC from the filter and shut off a protein skimmer before medicating fish.
Biological filtration works to eliminate the presence of ammonia and
nitrite in the aquarium water. Ammonia and nitrite probably account for
more health problems and deaths with aquarium fish than any other cause.
The fish excretes (directly through the gills) approximately 60 percent of the
ammonia in an aquarium. The remainder is produced by bacterial breakdown
of fish feces and urine, uneaten food, dead animals and plant byproducts.
Nitrite is a waste product of the bacteria that use the ammonia for food.
The accepted theory is that ammonia in aquarium water hinders
oxygen uptake in the fish's blood. Lethal concentrations cause chronic stress. Gill filaments
become irritated and begin to swell. This cuts off the oxygen supply to the
membrane cells, when they become infected with bacteria. Ammonia-related
stress also lowers a fish's resistance to parasite and bacteria attacks,
reduces food intake, and can result in death. An ammonia test kit measures
total ammonia, which in a well-managed aquarium should be immeasurable. All
aquarists should have an ammonia test kit and use it on a regular basis.
Likewise, Nitrite toxicity involves blocking oxygen uptake in the blood.
As nitrite levels increase, the concentration
of methemoglobin (as a percentage of total hemoglobin) goes up, reducing
the capacity of the blood to carry oxygen. Sub-lethal concentrations
produce chronic stress similar to ammonia. Here again, regular use of
a nitrite test kit is an absolute requirement for a fishkeeper. There are
two basic solutions to this problem: ion exchange and biological filtration.
Ion exchange is a chemical filtration process that removes ionized ammonia (NH4+)
from the water by swapping it for an ion in the exchange medium. Both natural
zeolites (which look like little chips of cement) and synthetic resins (which look
like little plastic beads or chips) are used to remove ammonia from aquarium water.
Ion-exchange resins (but not zeolites) also remove nitrites and nitrates.
The exchange medium is placed in a compartment of an outside filter
(after the compartment with GAC so that the water flows through the
GAC first) and aquarium water is forced through it. Ion-exchange media
can only be used in freshwater tanks. This also precludes the use of salt in
freshwater tanks for medicinal purposes when using ion exchange. Ammonia ion
exchange is also severely inhibited in hard water. Even moderate levels of
DOC in the water significantly reduce ammonia ion exchange. Thus, the use
of zeolites or ion exchange resins requires continuous filtration of
particulates and DOCs.
When ion-exchange material becomes saturated with ammonia, it must
be recharged or replaced. Otherwise, the ammonia concentration will quickly
build up to dangerous levels (known as ammonia break-through). Therefore,
the use of ion-exchange media requires regular and frequent ammonia testing.
An alternative approach is biological filtration,
a method that uses naturally occurring nitrifying bacteria to detoxify
nitrogenous wastes. One type of bacteria converts ammonia to nitrite.
These bacteria are usually well established in aquarium gravel, on tank
walls and on plant surfaces within two weeks of adding fish to a new setup.
Another naturally occurring bacteria converts the nitrite into a far less
harmful form of nitrogen waste: nitrate. Laboratory studies have shown that
nitrates are far less harmful to fish than once believed. Concentrations in
excess of 400 mg/L appear safe for fish. (Nitrates are more toxic to marine
Although all aquariums have some amount of natural biological filtration occurring on the surface
of the gravel bed, it is usually far too little to handle the fish load.
A dedicated biological filter is significantly more effective.
Strive to use a filter that has all three types of filtration in one unit.
See information about how to
choose the right type of filter for your tank.