Pet Forum / Aquaria / Marine Reef / December 2006

Nitrates in Saltwater Aquariums
By Jaime Rivera-Sierra, www.guayni.com

           Belief it or not, Nitrogen (N) and Phosphorous (P) are limiting
nutrients in the oceans.  This means that producers such as diatoms and
other algae can't produce more because of limited amounts of these two
elements.  This is the way that the oceans had evolved over millions of
years.  When there is an excess of N and P, the algae bloom causing red or
green tides.  This phenomenon instead of being helpful to the ocean's
inhabitants results detrimental by disrupting the entire food chain and the
ocean's equilibrium.  In open waters this should be avoided at all costs.

           On this paper I will discuss the effects of excess nitrogen in
saltwater aquariums in various forms.

           Nitrogen in a saltwater aquarium is present in four different
forms.  I have to emphasize that it enters your aquarium in the form of
food.

The most stable form of nitrogen in an aquarium is in the form of living
tissue.  Every living organism incorporates nitrogen to each and every cell
for many reasons.  The amount of nitrogen not needed for flesh is excreted
back to the water in the form of ammonia.

The ammonia enters the water column in the form of urine, feces or gills'
secretions.  A high level of ammonia in an aquarium is toxic to fish and
disrupts the other life cycles of your aquarium's residents.  Ammonia is the
most toxic and unstable of the four nitrogen forms.

           Once Ammonia enters the water column either evaporates as
ammonia gas (NH4+) or gets recycled in the nitrogen cycle.  This task is
performed by nitrifying bacteria that turns ammonia (NH3) to nitrites (NO2).
In the nitrite form, N stays unstable replacing oxygen in breathing and
affecting in many ways the metabolism of your fish.   Although not as
unstable as ammonia, nitrites will deteriorate your aquarium and can kill
your fish if levels exceed a tolerable margin.  High levels of nitrites must
be avoided.

           The nitrifying bacteria continue to process nitrogen-containing
compounds turning nitrites to nitrates (NO3).  At this point nitrogen is at
its lowest unstable condition becoming accessible to plants.  In the nitrate
form, algae and other plants reincorporate nitrogen into its tissue.  In
open waters, these plants are eaten by consumers restarting the cycle again.
On the other hand, in a closed system as your aquarium, nitrates accumulate
to detrimental levels.

An excess accumulation of nitrates may cause some problems to your system as
it remains unstable.  NO3 can't be reduced again by nitrifying bacteria and
has to be removed from the tank.  There are different methods to achieve
this task.  The most common procedure is known as a "water change".  Using
this technique, aquarists remove water from the tank and replace it with
waters with low nitrates levels, thus diluting the water column.  This
method is costly and tedious requiring periodical water changes that may
range from twice a year to twice a week and from 10% to 20% of the water
column.  Another good method of extracting nitrates from the water is by
harvesting plants that incorporated nitrates to its tissue.  Since under
this conditions N is not a limiting nutrient, plants will bloom and algae
will cover your tank making it easy to remove.  The more plants you harvest,
the less need for periodical water changes.  Each plant you extract contains
nitrogen built into its living tissue from the most available source, NO3.

In summary, nitrogen is an element that needs to be controlled in your
system.  The nitrogen cycle works only one way.  The nitrogen in your system
enters in the form of food, this food is taken by your fish and the excess
is redirected to the water column in the form of ammonia.  Nitrifying
bacteria turns ammonia to nitrites and then to nitrates.  Once the nitrogen
is in the form of nitrates the only way out is mechanical.  An aquarist has
to make a decision to extract nitrates directly from the water by means of
water changes or by harvesting plants that utilized nitrates as nutrient.

References;

Dakin, N. 2003. Complete Encyclopedia of the Saltwater Aquarium

Mitsch, W.J. and J.G. Gosselink. 2000.  Wetlands 3rd Ed.

Rivera-Sierra, J.  1992.  Impacto Ambiental Acumulativo Del Desarrollo
Costanero En El Sector Playita De Salinas, Puerto Rico