Every new aquarium set-up goes through a process of establishing
beneficial bacterial colonies. This process is generally known as
the nitrogen cycle, but is also called nitrification, the start-up
cycle and/or the break-in cycle. As you know, your aquarium is a
closed environment so all of the waste excreted from your fish and
uneaten food accumulates in your aquarium. The nitrogen cycle
converts these wastes to safer by-products. The fluctuations you
have noticed in your aquarium water quality recently are likely the
result of this cycle.
The cycle begins when you add fish to the aquarium. All uneaten,
decayed food and waste generated by the fish breaks down to form
ionized or unionized ammonia. The ionized form, Ammonium (NH4), is
present if the pH is below 7, and is not toxic to fish. The
unionized form, Ammonia (NH3), is present if the pH is 7 or above,
and is highly toxic to fish. These ammonia levels will increase for
about 2 weeks until the second stage of the cycle begins. (The
Freshwater Master Test Kit's ammonia test gives a combined reading
of Ammonium (NH4) and Ammonia (NH3).)
During the second stage of the nitrogen cycle aerobic bacteria
called nitrosomonas grow to sufficient quantities in the filter to
convert the ammonia to toxic nitrite. (Nitrite destroys the
hemoglobin in the fish's blood and eventually prevents the blood
from carrying oxygen) As this happens, the ammonia levels will
quickly begin to drop as the nitrite levels slowly increase. These
nitrite levels will continue to increase for about 2 weeks until
aerobic bacteria called nitrobacters grow to sufficient quantities
in the filter to convert the nitrite to less toxic nitrate.
If your current water quality testing indicates high nitrites, the
nitrobacters are still establishing themselves in your filter media,
gravel and hydro-sponge.
The conversion of nitrites to nitrates is Stage 3 of the nitrogen
cycle. Again, as the nitrite levels quickly decrease due to the work
of nitrobacters, the nitrate levels will slowly increase. Once your
tank has reached this point (about 5-6 weeks total), it is said to
have "cycled". All you need to do now, is to perform your regular
partial water changes in order to keep a moderately low nitrate
level. If this practice is followed routinely, you should have no
problems maintaining your biological filter.
What not to do during the nitrogen cycle:
-Don't change the filter media – the beneficial bacteria are growing
there. Don't disturb them until they have become well established.
(You may need to clear debris from the filter sponge once a week--
do this in dechlorinated water.)
-Don't overfeed the fish – when in doubt underfeed your fish.
Remember that anything going into the tank will produce waste one
way or another.
by Emily Gates, Pennsylvania Council of Trout Unlimited
in consultation with this reference
Many TIC tanks now use MicrobeLift Special Blend, which further converts nitrates into nitrogen gas, which then bubbles out of the tank. Some water changes and vacuuming are still advisable, but on the whole nitrate levels are lower and more manageable with this product.