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Nitrogen Cycle: The Key to Biological Filtration
Veterinary & Aquatic Services Department, Drs.
Foster & Smith, Inc.
Understanding the nitrogen cycle is an important part of keeping a
successful aquarium. The nitrogen cycle is responsible for the
biological filtration within the system. It keeps the water free of
toxic compounds that are a result of the respiration of the
inhabitants, and the decay of any matter such as waste products and
uneaten food. When we understand this cycle, we can anticipate
situations that may cause damage to this process, and prevent or avoid
these situations that may lead to livestock loss.
What is the nitrogen cycle?
Figure 1. The
nitrogen cycle
Adapted from: Mills, D. The
Marine Aquarium. Salamander Books LTD.
8 Blenhein Ct., Brewery Rd. London N79NT; 1987. |
In the nitrogen cycle, the waste products of the fish, plants, and
invertebrates, along with any dead organisms or uneaten food, are
broken down by bacteria and fungi into the resulting chemical,
ammonia. Ammonia is extremely toxic to all of the aquarium
inhabitants. It is broken down by an oxygen-loving bacteria, Nitrosomonas.
The Nitrosomonas* bacteria feed on both oxygen and ammonia,
and with their biological activities, they excrete a chemical called
nitrite. Although nitrite is not as toxic as ammonia, even at low
concentrations in the aquarium, it can be harmful to fish and
invertebrates. Another bacteria Nitrobacter*, which also
utilizes oxygen in its respiration, acts in a similar way as Nitrosomonas,
and essentially changes the nitrites into a relatively harmless
chemical called nitrate. The bacteria that will feed on nitrates are
anaerobic, meaning they grow in areas of little or no oxygen. They
require low-oxygenated stagnant water, and can be found in more
elaborate filtration systems and within live rock. Here they breakdown
nitrates into free nitrogen.
* Note: Recent research by one of the leading
companies in the aquatics industry has found that other bacteria (some
of which are still unnamed) are involved in the nitrogen cycle. The
company has isolated these strains of bacteria in the freshwater
systems and will be marketing them in a product to be used as a
cycling aid. A product containing the saltwater strains is still in
development, but is expected to be available in the near future. In
this article, I have used the names of the bacteria commonly referred
to in current textbooks and journals, understanding these names may be
added to or changed as we learn more. For home aquarists, the names of
the specific bacteria are not as important as understanding the
process and what may affect its efficiency.
The nitrogen cycle in new aquariums
Newly-set-up aquariums lack the colonies of bacteria that are
necessary to perform the biological filtration. Because of this, the
aquarium must be "cycled." "Cycling" refers to the
process of establishing and maturing the biological filtration. In
order to establish the system, we need to provide a source of ammonia
for the Nitrosomonas bacteria in the filtration system so
they can live, reproduce, and colonize. To provide an ammonia source,
it is best to add a few hardy fish that can withstand the presence of
ammonia and nitrites. Then we need to seed the aquarium with bacteria.
There are commercially available cycling aids that contain the
bacteria. Otherwise, when purchasing the hardy fish, request a small
amount of gravel from the aquarium where the fish were held. This
gravel should then be placed in the new aquarium along with the fish,
and will provide the bacteria that are necessary to seed the system.
Figure 2 illustrates the process that occurs in an aquarium during the
biological filter maturing process.
As the fish in the new system are fed and begin to thrive, they
will, through their biological activities, produce ammonia. The Nitrosomonas
bacteria, in turn, will begin to feed upon that ammonia and will start
populating the aquarium. Their population will be greatest in the
media that contains the highest level of oxygen and surface area,
which will normally be within the filtration system. At this point,
because the numbers of bacteria are limited, they will not be able to
convert all of the ammonia that is present in the system, so the
ammonia levels will continue to rise. As the amount of ammonia
increases, the population of bacteria will also increase, but at a
much slower rate than the ammonia. The ammonia level will eventually
reach a peak and then start to decline as the population of bacteria
becomes large enough to break down the ammonia faster than it is being
produced. Because there is still ammonia within the system, however,
the bacteria will continue to live and feed on the ammonia until it
reaches a level undetectable by testing. At this point, a balance has
been achieved in which the rate of ammonia production equals the rate
at which it is broken down by the bacteria. The number of bacteria,
from this point on, will change as the levels of ammonia (their food
source) changes.
Figure 2.
Biological filtration cycling process
Adapted from: Mills, D. The
Marine Aquarium. Salamander Books LTD.
8 Blenhein Ct., Brewery Rd. London N79NT; 1987. |
As we can see in Figure 2, the nitrites go through a very similar
cycle as the ammonia. Nitrites are produced through the biological
activities of the Nitrosomonas bacteria as they feed on the
ammonia. As their numbers increase, so does the amount of their waste
product, nitrites. The Nitrobacter bacteria, because of the
increasing supply of nitrites, will multiply and increase in numbers.
They, too, will be most densely populated in the area with the
greatest surface area and oxygen content. The nitrite levels will rise
until the number of bacteria has increased to the point at which they
break down the nitrites faster than it is being produced. At this
point, the peak level of nitrites has occurred, and the bacteria will
continue to metabolize and feed upon the nitrites that are produced.
The nitrite level will decrease until it becomes undetectable. As with
the Nitrosomonas, the Nitrobacter will constantly
alter their numbers as the amount of nitrites changes, keeping a
balance at which the nitrites are undetectable.
The end product of this whole process is nitrate. Nitrates, in low
to moderate concentrations, are not toxic to fish and invertebrates.
Nitrates, however, can serve as a nutrient source for bacteria and
plant life, and be the cause of other problems in the aquarium, such
as excess algae. The anaerobic bacteria will break down the nitrates.
Plants within the system will also feed on nitrates and are a good
natural way of controlling this nutrient. Otherwise, the nitrate level
needs to be controlled by chemical filtration and partial water
changes.
The length of time required for this cycle to be completed in the
new aquarium depends on many factors. These factors include: the
amount of ammonia being produced during the cycling period; the
efficiency of the biological filtration; and whether live rock or live
plants are used in this process. The typical time period in most
aquariums is going to be 3 to 6 weeks. It is important that if any of
the fish used during this process perish, that they be replaced with
another hardy fish in order to maintain the input of ammonia.
The nitrogen cycle in established aquariums
An established aquarium is one that the biological filtration has
been matured. There are situations, however, that affect the nitrogen
cycle in established aquariums, such as: adding livestock; unnoticed
death in the aquarium; overfeeding; medicating the aquarium; and
system maintenance.
Adding livestock
In the biological filter of an established aquarium, there are just
enough bacteria to handle the biological load that is placed on the
system at that time. When we add livestock to this system, we are
increasing the amount of ammonia for the bacteria in the biological
filter to metabolize. This situation brings us back to the cycling
process (Figure 2), where the bacteria begin to multiply to make up
for the extra biological load. How high the toxins will become in the
system is going to depend both on the amount of livestock added to the
aquarium at one time, and the size of the aquarium. If too much
livestock is added at one time, it is possible for the ammonia and
nitrites to reach dangerous levels, which may lead to livestock
losses. It is important to minimize these levels by stocking the
aquarium slowly over time, giving the biological filtration time to
catch up to the load.
Situations may arise where it is advantageous to stock the aquarium
at a faster rate than the biological filtration can handle. These
situations include:
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 Buying
livestock through the mail: Because of the price of
shipping, most aquarists will place large orders when purchasing
their livestock through a mail order company. Although they may
offset the shipping cost in doing this, they are also placing the
new and existing livestock in jeopardy.
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Adding many aggressive fish at the
same time: It is best to acclimate aggressive fish of the
same species at the same time. Doing this will allow the fish to
set their own territories prior to one individual taking over the
entire aquarium.
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Transferring livestock to a hospital
tank: The hospital tank is typically a small aquarium with
minimal filtration. Levels of toxins must be monitored closely
when any number of fish are added to this new system.
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Unnoticed death in the aquarium
It is possible in many aquariums, such as freshwater planted and
saltwater reef aquariums, to have an inhabitant perish in a place
where it cannot be seen. When this happens, the organism begins to
decay, which places a large load on the biological filtration. Again,
the nitrogen cycle can be thrown out of balance depending on both the
amount of death in the system, and the size of the aquarium. Having a
large aquarium, in this case, is advantageous because the ammonia
being produced by the organism will be diluted by the large volume of
water.
Overfeeding
When feeding the aquarium, it is important that the food that is
added for the fish and invertebrates is consumed within a short period
of time. After a few hours, any food that is left uneaten in the
aquarium will begin to be broken down by the bacteria and fungi,
resulting in ammonia added to the system. This ammonia in turn becomes
part of the biological load and if the amount of decaying food is
great enough, can cause an imbalance in the biological filtration. If
the aquarium has been overfed, it is necessary to siphon out any
uneaten food and to perform a 25% water change.
Medicating the aquarium
Many medications affect the ability of the bacteria to function in
the biological filtration. For instance, anti-bacterial medications
act in the way the name describes, by killing many types of bacteria.
Unfortunately, the biological filtration is bacteria-based, and will
be affected by these medications. Other medications such as copper,
antibiotics, and ich treatments will also affect the filtration in
different degrees. It is important, when treating an aquarium, to
closely monitor both the ammonia and nitrite levels and to perform
water changes or chemical filtration when necessary.
System maintenance
Water changes and filter maintenance will both affect the
biological filtration to some degree. When performing water changes,
it is important that the replacement water is free of any toxic
chemicals such as chlorine. These chemicals can kill bacteria within
the system and any water that is to be used, should be treated either
by reverse osmosis, or by one of the many available liquid
dechlorinators. Filter maintenance, if not done properly, can have a
large effect on the biological filtration. Again, the beneficial
bacteria responsible for the nitrogen cycle, populate in the greatest
numbers where the water flow and oxygen content of the water are the
highest. This is typically within the filter. When performing
maintenance on the filter, it is ideal to leave the biological media
untouched in order to preserve the bacteria. If there is no biological
media within the filter, it is wise to change only ½ of the
mechanical media at a time. The remaining media that is to be reused
should be rinsed in water taken from the aquarium in order to preserve
the bacteria colony.
Restoring the balance
All of the above situations can cause an imbalance in the nitrogen
cycle, and make it necessary for us to monitor the level of toxins in
the system whenever they occur. If any level of either ammonia or
nitrites is detected, it is important to control these toxins either
through partial water changes, or with one of the available
toxin-absorbing resins.
When performing water changes, it is important to change no more
than 25% of the aquarium water at a time. Changing more than 25% of
the aquarium water can cause rapid changes in both temperature and pH,
which can result in added stress to the aquarium inhabitants.
Therefore, if toxins are present, it is best to perform small water
changes frequently (even daily) rather than performing large water
changes at less frequent intervals. Again, the makeup water that is
used to replace the aquarium water should be treated by reverse
osmosis, distillation, or at the very least using a liquid
dechlorinator. It is ideal that the makeup water is at the same
temperature as the aquarium, and has been aerated prior to adding it
to the aquarium.
There are many chemical medias available on the market that will
help control sudden increases in ammonia. By stopping the ammonia
prior to it being broken down by the bacteria, we are reducing the
biological load on the system. These products can be useful in the
situations that have been described above. Again, it is important when
using these products to monitor the water quality, and to perform
water changes when any toxin levels are detected.
Know the warning signs
It is not practical to constantly test and monitor our water for
ammonia and nitrites, but there are signs that we can see within the
aquarium. These signs are the actions of the fish and invertebrates.
When ammonia or nitrites are present in the water, the fish will show
signs of stress. These signs can be in the form of erratic swimming
behavior, gasping, or even laying on the substrate.
These activities can also be the sign of disease, but our first
reaction should be to test the water for ammonia and nitrites.
Likewise, invertebrates such as corals will also show signs of
distress. These signs are represented by the poor expansion of the
coral, losses in coloration, and deterioration. If these signs do show
themselves within the aquarium, it is then important to test the water
for any amounts of ammonia or nitrites. If any levels are detected,
then we must correct the situation by the methods that I have
described above.
Conclusion
Maintaining a healthy aquarium starts with understanding the
nitrogen cycle and its effect on the inhabitants. This cycle takes
time to stabilize the water conditions both in the initial set-up, and
after adding livestock. It is important to stock your new aquarium
slowly and to allow the cycle to be completed prior to adding any new
inhabitants. If you understand this process, pay attention to the
warning signs, and take appropriate actions, there is no reason for
catastrophic die offs in the aquarium due to ammonia or nitrites.
References
Delbeek, JC; Sprung, J. The Reef Aquarium Vol. 1. Ricordea
Publishing. Coconut Grove, FL; 1994.
Mills, D. The Marine Aquarium. Salamander Books LTD. 8 Blenhein
Ct., Brewery Rd. London N79NT; 1987.
© 2005 Drs. Foster
and Smith, Inc.
Reprinted as a courtesy and with permission from
PetEducation.com (http://www.PetEducation.com)
On-line store at http://www.DrsFosterSmith.com
Free pet
supply catalog: 1-800-323-4208
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