The Importance of the Nitrogen Cycle

If you are thinking of installing a custom aquariums system, the most dangerous
water pollutant that you should be aware of is ammonia. This toxin is produced by the
waste expelled by fish and other decaying organic matter. Without proper filtration, the
amount of ammonia will rise to toxic levels, eventually killing all livestock. The process
of changing ammonia into non-toxic substances is known as the process of nitrification
and is achieved primarily through biological filtration. The goal of nitrification is to
convert ammonia into something harmless and non-toxic.

How the Nitrogen Cycle Works

The first step in the nitrogen cycle begins as soon as livestock are first introduced
into an aquarium. Leftovers from feeding the fish, waste, and other organic material, is
eaten by bacteria or fungi and converted into ammonia. This is known as the process of
ammonification. As ammonia levels increase, a species of nitrifying bacteria, known as
Nitrosomonas, break down the ammonia into nitrite. Soon, the ammonia levels begin to
fall and the levels of nitrite begin to rise. When the nitrite levels go up another species
of nitrifying bacteria, known as Nitrospira, converts the nitrite into nitrate. In nature the
process of nitrification continues in two ways. Some of the nitrates are absorbed by
plants. These plants are in turn eaten by organisms, which then release organic waste.
The waste is then converted into ammonia through the process of ammonification,
thereby completing the nitrogen cycle. Also, some excess nitrates are consumed by
anaerobic bacteria. They convert the nitrates into nitrogen gas which escapes into the
atmosphere.

Biological Filtration: The key to successful nitrification.

Needless to say, it is of the utmost importance that the process of nitrification
takes place in every aquarium. Otherwise, the fish will literally poison themselves to
death, by swimming in their own toxic waste. Let’s look more closely at what is
necessary to get this process started, and keep it going. The most basic element in the
process of nitrification is the nitrifying bacteria. If you can maintain a healthy amount of
all the species of bacteria necessary to complete the nitrogen cycle you will successfully
keep your water toxin free. So, you might be wondering how you are going to find all
these kinds of bacteria to keep in you aquarium. Well, luckily for us fish-keepers, the
bacteria will grow naturally in your aquarium as long as they find a suitable
environment. The preparation of an environment to support the growth of nitrifying
bacteria is called biological filtration. The bacteria require two things in order to
establish a population in your aquarium.


Requirement One: A place for the bacteria to grow.

The first requirement is a home for the bacteria grow and thrive in. The key
element in a bacterial home is the surface area. In freshwater aquariums the bacteria
will grow in the substrate, but this will not usually provide enough space to grow an
adequate population of bacteria to deal with the waste of the fish population. Instead
the health of the aquarium is dependent on the biological filter. Most canister filters
contain layers of pebbles or other materials which have a large surface area. This is to
provide a home for the bacteria to grow in. For large aquariums, or, if a large population
of fish is desired, a wet-dry filter is usually required. The wet-dry filter has the water drip
through a compartment of bio-balls which are specifically manufactured to provide a
tremendous amount surface area for the bacteria to grow on. Additionally, in a wet-dry
filter the bio-balls are not submerged in water. Instead, the water drips through the bioballs
(hence the name wet-dry) which provides a nice moist atmosphere for the bacteria
to grow in. In a saltwater aquarium the live rock and live sand has a large surface area
and can sometimes suffice to grow an adequate population of bacteria. Nevertheless, in
large marine aquariums, especially in custom aquarium systems a wet-dry filter is
usually added to increase the amount of fish that can be safely kept in the aquarium.

Requirement Two: A constant supply of nutrients.

The second requirement is a food supply. Nitrosomonas will require a constant
supply of ammonia to be converted into nitrite. The Nitrospira will require a constant
supply of nitrite to convert into nitrate. Without a supply of these nutrients the
nitrifying bacteria will not grow. We have explained how to provide a home for your
bacteria with a biological filter. Now, let’s look at the process of building up a supply of
nutrients for the bacteria in a new aquarium.

Aquarium Cycling: What to expect when you start your new aquarium.

When your aquarium is filled for the first time, there is very little organic
material in the water. Therefore the small amount of bacteria that is in the water will
not multiply because they do not have a source of food. At this stage it is crucial not to
place too many fish in the aquarium because there is no bacterial population to perform
the nitrification process. No matter how big your biological filtration is, because there is
no bacterial population living there yet, the waste will not be broken down into nontoxic
substances. The ammonia levels will go up and the fish will die. This is like having a
huge water-treatment plant to clean a cities drinking water, but if no one is working in
the plant operating the machinery the water will not be cleaned. So, how do you get the
bacteria to begin growing in the environment that you prepared for them? This is a
three step process.

Step One: Ammonia

First you should place one or two small fish in the aquarium. These fish will
produce waste which will be converted into ammonia. This will cause the ammonia
levels to spike. The population of nitrifying bacteria that converts ammonia into nitrites


(Nitrosomonas) will grow because they now have a source of food. Over a period of
about three weeks the ammonia levels will gradually fall until they reach zero.

Step Two: Nitrite

At the same time as the ammonia levels are falling, the nitrite levels will rise,
because the population of bacteria that consume nitrite (Nitrospira) has not yet grown
to full strength. While nitrite is less toxic to fish than ammonia, it is still a toxic
substance that must be removed. After four to six weeks the nitrite levels will slowly go
down to zero as the population of Nitrospira grows.

Step Three: Nitrate

I’m sure you already guessed that the falling of the nitrite levels will cause a
spike in the nitrate levels in the aquarium. (If you don’t understand this see Figure 1
below.) Low levels of nitrates are not toxic for fish, but they do cause unattractive algae
growth and high levels of nitrates have been attributed as the cause of some fish
diseases. As we discussed before, in nature the process continues when plants consume
the nitrates, and, when anaerobic bacteria convert the nitrates into nitrogen gas and
release it into the atmosphere. In freshwater live plant aquariums the plants will
consume some of the nitrates, also in saltwater marine aquariums there might be
pockets of anaerobic bacteria living in the live rock that convert the nitrates into
nitrogen gas which rises to the surface of the aquarium and is released harmlessly into
the atmosphere. However, both of these scenarios will rarely remove the nitrates from
the water as fast as the nitrifying bacteria can produce it. Therefore, the nitrate level
will continue to rise. Fish-keepers deal with this problem through periodic water
changes. A 15% to 20% water change every two weeks will usually suffice to keep the
nitrate levels within safe limits. The amount of your water change should depend on the
nitrate levels. If there is a high level of nitrates in the aquarium, perform a larger water
change. If the nitrate levels are low even a 10% water change is fine. Nitrate levels
below 20 PPM are ideal, but, levels below 40 PPM are still perfectly safe for your fish. In
all aquariums ammonia and nitrite levels should be kept at zero.


After The Initial Cycle is Completed

As the population in your biological filter increases you can slowly add more fish.
Each fish added will increase the waste in the water, which will in turn increase the
populations of all the species of bacteria necessary to break down the waste into
nitrate. Depending on the size of the aquarium it can take up to six months before the
maximum biological load that the filtration system can handle is reached. The key is to
take it slow. By far the most common cause of failure among new aquarium hobbyists is
putting in too many fish too fast, causing the toxin levels to rise way past what the
nitrifying bacteria can handle. This will poison the water and can cause the death of all
the fish in the aquarium! As a fish-keeper you are investing enormous amounts of time,
effort, and money, you owe it to yourself to ensure that your investment is not in vain.
Be patient in the beginning and you will create a balanced eco-system that will grow
with you for many years.