Saturday, August 16, 2008

The Nitrogen Cycle

Many will agree that understanding the Nitrogen cycle is one of the most beneficial pieces of information that a fish keeper could know. Unfortunately, it is also one of the most misunderstood aspects of the hobby, especially by beginners. Its quite understandable, the Nitrogen cycle can be a hard concept to understand with all the different types of bacteria, the different process, byproducts and effects that it has on the fish and other aquatic creatures. The Nitrogen cycle is important because it is the basis for balance within our mini ecosystems, without this balance, the system will likely fail. In simple terms, it transforms the toxic levels of Ammonia (NH3) and Nitrite (NO2-) into a much less form of Nitrogen known as Nitrate (NO3-).

The cycle begins with fish waste, excess food, and other dissolved organics decomposing and creating the byproduct Ammonia. Ammonia is toxic to fish, especially at higher Ph levels, and high temperatures. Keep in mind that, although less toxic at a lower Ph, it is still a very deadly element to fish. At this stage in the cycle, two things can happen to the Ammonia. The first being that is it taken up by plants, being that it is the easiest form of Nitrogen for plants to consume, the plants will take advantage of any source of ammonia present. The second path that the Ammonia can take involves a bacterium called Nitrosomonas. Nitrosomonas consume the Ammonia and when in aerobic (the presence of oxygen) conditions, converts the Ammonia into a similarly toxic substance known as NO2- or Nitrite. Nitrite, unlike Ammonia is toxic to plants as well as fish. The Nitrite is then consumed by a bacterium known as Nitrospira, creating a byproduct known as Nitrate. Nitrate is a much less toxic form of Nitrogen than Ammonia and Nitrite. Although it is less toxic, it still poses a threat to fish if the levels are allowed to accumulate to high levels. Ideally, Nitrate levels should be as low as possible.

Removing water on a regular basis and replacing it with new, clean water is an excellent way to keep the Nitrate from reaching toxic levels. Another method is to include live plants in your setup, whether they be directly in the main tank or in a sump/refugium. A medium to heavy planted tank shouldn't show a significant (>5 ppm) Nitrate reading.

Cycling a Tank

The process of cycling a tank is defined in simple terms as providing a constant source of Ammonia to the tank and waiting for the beneficial bacteria to build up. A cycle can take anywhere from a week to several months to complete. There are several methods used by Aquarists to cycle tanks, some of which are outlined below. Water changes is something that should be avoided during the cycling stages. By replacing water, the Aquarist could be disrupting the processes that are going through, lengthening the amount of time the cycle will take. A cycle consists of a spike in Ammonia, followed by a spike in Nitrite, and then a rise (sometimes considerable) in Nitrates. The cycle is complete once the Ammonia and Nitrite levels have settled at 0.

Cycling with Fish

Cycling with fish is likely the most common way that a tank is cycled, especially in new hobbyists. This method is considered unethical, and is not supported by many Aquarists including myself for a number of reasons. For the sake of this article, which is intended to inform those on the science and methods of cycle I will not go into detail about ethics.

Basically, the Aquarist stocks the tank with a small number of rather hardy fish. These fish produce a steady and consistent amount of ammonia each day (One of the few upsides). After the tank has cycled, these fish are often removed and replaced with other fish. The first and foremost downside is that the fish are subjected to toxic levels of Ammonia, and Nitrite. Some recommend doing daily water changes to keep the levels low and the fish alive. Unfortunately, by removing the water, they are also removing the Ammonia that is needed to cycle the tank. This method is very simple, but subjects the fish to stress and puts them in deadly situations.

Fishless Cycling

This method is really catching on with hobbyists as one of the best ways to cycle a tank. Rather than adding fish to create an Ammonia source, the hobbyist adds it through other methods. Some of these methods include adding pure ammonia at a consistent amount daily (enough to create around 4 ppm) everyday, or adding a dead prawn. These sources of Ammonia will start the cycle, and continue to feed it throughout the process. Again once ammonia and Nitrite levels reach 0 (Measure more than 24 hours after your most recent dose of ammonia, measuring before this will clearly give you an ammonia reading) the tank is cycled. This method is better because it doesn’t subject fish to undue stress, it is also cheaper but equally as effective.

Silent Cycling

Silent cycling is a method of cycling that involves using a large amount of plants to handle the cycle. Since Ammonia and Nitrates are readily consumed by plants, this allows the Aquarist to stock the tank with a fair amount of fish very early on. It is called Silent cycling because the plants consume any Ammonia that is produced by the fish, causing no further spikes in any of the compounds. To perform this is simple, simply plant a tank at least with a moderate amount of plants (Plants appear to take up more than 40% of the tank) and add some of your planned fish. Essentially, the plants act as the bacteria, while the actual bacteria are colonizing.

Instant Cycling

Instant cycling is possibly one of the best ways to setup a new tank, it’s the fastest, cheapest and most efficient way of getting the bacteria colonies established. To instant cycle a tank, simply introduce either some filter media, or substrate that has been in a fully established system for at least 2 weeks. These “soiled” medias will be hosting a sufficient number of bacteria to start and stock your new tank almost instantly. I recommend using this method whenever possible.


Cycling a tank is the single most important part of setting up a tank, and should be done with care and done correctly. If done incorrectly, it can lead to death, disaster, and money out of your pocket. Mind you, when using any of the methods listed above, it is not recommended to fully stock the tank with all the fish that have been planned for it, instead the Aquarist should always take it slowly, adding only a few fish at a time. Of course some methods will allow the Aquarist to stock a larger amount of fish initially, it still should not be the full amount. As you can see, each method is only briefly outlined, in the future, Aquatic Revolution will feature more in depth articles on each of these methods.