28 What Is the Nitrogen Cycle?
Description
This article is from the Beginning Fishkeeping FAQ, by Thomas Narten with numerous contributions by others.
28 What Is the Nitrogen Cycle?
Like all living creatures, fish give off waste products (pee and poo).
These nitrogenous waste products break down into ammonia (NH3), which
is highly toxic to most fishes. In nature, the volume of water per
fish is extremely high, and waste products become diluted to low
concentrations. In aquariums, however, it can take as little as a few
hours for ammonia concentrations to reach toxic levels.
How much ammonia is too much? The quick answer is: if a test kit is
able to measure it, you've got too much (i.e., it's in a high enough
concentrations to stress fish). Consider emergency action (water
changes and zeolite clay) to reduce the danger. (A more detailed
discussion of ammonia toxicity can be found later in this section.)
In aquaria-speak, the ``nitrogen cycle'' (more precisely, the
nitrification cycle) is the biological process that converts ammonia
into other, relatively harmless nitrogen compounds. Fortunately,
several species of bacteria do this conversion for us. Some species
convert ammonia (NH3) to nitrite (N02-), while others convert nitrite
to nitrate (NO3-). Thus, cycling the tank refers to the process of
establishing bacterial colonies in the filter bed that convert ammonia
-> nitrite -> nitrate.
The desired species of nitrifying bacteria are present everywhere
(e.g., in the air). Therefore, once you have an ammonia source in your
tank, it's only a matter of time before the desired bacteria establish
a colony in your filter bed. The most common way to do this is to
place one or two (emphasis on one or two) hardy and inexpensive fish
in your aquarium. The fish waste contains the ammonia on which the
bacteria live. Don't overfeed them! More food means more ammonia! Some
suggested species include: common goldfish (for cold water tanks),
zebra danios and barbs for warmer tanks, and damselfishes in marine
systems. Note: Do not use ``toughies'' or other feeder fishes.
Although cheap, they are extremely unhealthy and using them may
introduce unwanted diseases to your tank.
During the cycling process, ammonia levels will go up and then
suddenly plummet as the nitrite-forming bacteria take hold. Because
nitrate-forming bacteria don't even begin to appear until nitrite is
present in significant quantities, nitrite levels skyrocket (as the
built-up ammonia is converted), continuing to rise as the
continually-produced ammonia is converted to nitrite. Once the
nitrate-forming bacteria take hold, nitrite levels fall, nitrate
levels rise, and the tank is fully cycled.
Your tank is fully cycled once nitrates are being produced (and
ammonia and nitrite levels are zero). To determine when the cycle has
completed, buy appropriate test kits (see the TEST KIT section) and
measure the levels yourself, or bring water samples to your fish store
and let them perform the test for you (perhaps for a small fee). The
cycling process normally takes anywhere from 2-6 weeks. At
temperatures below 70F, it takes even longer to cycle a tank. In
comparison to other types of bacteria, nitrifying bacteria grow
slowly. Under optimal conditions, it takes fully 15 hours for a colony
to double in size!
It is sometimes possible to speed up the cycling time. Some common
procedures for this are detailed later in this section.
Warning: AVOID THE TEMPTATION TO GET MORE FISH UNTIL AFTER YOUR TANK
HAS FULLY CYCLED! More fish means more ammonia production, increasing
the stress on all fish and the likelihood of fish deaths. Once ammonia
levels reach highly stressful or toxic levels, your tank has succumbed
to ``New Tank Syndrome''; the tank has not yet fully cycled, and the
accumulating ammonia has concentrations lethal to your fish.
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