The Aqua Den

[JanS]

Understanding The Nitrogen Cycle

One of the most important aspects of starting a new aquarium is understanding the nitrogen cycle.
The nitrogen cycle is responsible for the biological filtration within the system. It keeps the water free of toxic compounds that are a result of the respiration of the inhabitants, and the decay of any matter such as waste products and uneaten food. When we understand this cycle, we can anticipate situations that may cause damage to this process, and prevent or avoid these situations that may lead to fish loss.

One of the biggest misconceptions is that people think letting the tank sit for a day or two after filling it is cycling. This is not cycling, and will do nothing except balance out your water temperature and let particles settle in your tank. The cycling doesn’t start until your ammonia source is added; whether it be fish or ammonia for fishless cycling.

What is the nitrogen cycle?
In the nitrogen cycle, the waste products of the fish, plants, and invertebrates, along with any dead organisms or uneaten food, are broken down by bacteria and fungi into the resulting chemical, ammonia. Ammonia is extremely toxic to all of the aquarium inhabitants. It is broken down by an oxygen-loving bacteria, Nitrosomonas. The Nitrosomonas bacteria feed on both oxygen and ammonia, and with their biological activities, they excrete a chemical called nitrite. Although nitrite is not as toxic as ammonia, even at low concentrations in the aquarium, it can be harmful to fish and invertebrates. Another bacteria Nitrobacter, which also utilizes oxygen in its respiration, acts in a similar way as Nitrosomonas, and essentially changes the nitrites into a relatively harmless chemical called nitrate.

The nitrogen cycle in new aquariums
Newly set up aquariums lack the colonies of bacteria that are necessary to perform the biological filtration. Because of this, the aquarium must be "cycled." "Cycling" refers to the process of establishing and maturing the biological filtration. In order to establish the system, we need to provide a source of ammonia for the Nitrosomonas bacteria in the filtration system so they can live, reproduce, and colonize.

As the fish in the new system are fed and begin to thrive, they will, through their biological activities, produce ammonia. The Nitrosomonas bacteria, in turn, will begin to feed upon that ammonia and will start populating the aquarium. Their population will be greatest in the media that contains the highest level of oxygen and surface area, which will normally be within the filtration system. At this point, because the numbers of bacteria are limited, they will not be able to convert all of the ammonia that is present in the system, so the ammonia levels will continue to rise. As the amount of ammonia increases, the population of bacteria will also increase, but at a much slower rate than the ammonia (this is when you get the almost white, cloudy water, or bacterial bloom). The ammonia level will eventually reach a peak and then start to decline as the population of bacteria becomes large enough to break down the ammonia faster than it is being produced. Because there is still ammonia within the system, however, the bacteria will continue to live and feed on the ammonia until it reaches a level undetectable by testing. At this point, a balance has been achieved in which the rate of ammonia production equals the rate at which it is broken down by the bacteria.

The nitrites go through a very similar cycle as the ammonia. Nitrites are produced through the biological activities of the Nitrosomonas bacteria as they feed on the ammonia. As their numbers increase, so does the amount of their waste product, nitrites. The Nitrobacter bacteria, because of the increasing supply of nitrites, will multiply and increase in numbers. They, too, will be most densely populated in the area with the greatest surface area and oxygen content. The nitrite levels will rise until the number of bacteria has increased to the point at which they break down the nitrites faster than it is being produced. At this point, the peak level of nitrites has occurred, and the bacteria will continue to metabolize and feed upon the nitrites that are produced. The nitrite level will decrease until it becomes undetectable. As with the Nitrosomonas, the Nitrobacter will constantly alter their numbers as the amount of nitrites changes, keeping a balance at which the nitrites are undetectable.

The end product of this whole process is nitrate. Nitrates, in low to moderate concentrations, are not toxic to fish and invertebrates. Nitrates, however, can serve as a nutrient source for bacteria and plant life, and be the cause of other problems in the aquarium, such as excess algae. The anaerobic bacteria will break down the nitrates. Plants within the system will also feed on nitrates and are a good natural way of helping to control this nutrient. Regular water changes are the best way to keep nitrates in check, whether you have live plants or not. Live plants will help, but there is no substitute for regular partial water changes.
The length of time required for this cycle to be completed in the new aquarium depends on many factors.
These factors include:
*The amount of ammonia being produced during the cycling period
*The efficiency of the biological filtration
*Whether live plants or established media are used in this process

The typical time period in most aquariums is going to be 3 to 6 weeks.

The nitrogen cycle in established aquariums
An established aquarium is one that the biological filtration has been matured. There are situations, however, that affect the nitrogen cycle in established aquariums, such as: adding fish; an unnoticed death in the aquarium; overfeeding; medicating the aquarium; and over cleaning your tank surfaces and media.

Adding aquatic life
In the biological filter of an established aquarium, there are just enough bacteria to handle the biological load that is placed on the system at that time. When we add fish to this system, we are increasing the amount of ammonia for the bacteria in the biological filter to metabolize. This situation brings us back to the cycling process (see separate articles), where the bacteria begin to multiply to make up for the extra biological load. How high the toxins will become in the system is going to depend both on the amount of fish added to the aquarium at one time, and the size of the aquarium. If too many fish are added at one time, it is possible for the ammonia and nitrites to reach dangerous levels, which may lead to losses. It is important to minimize these levels by stocking the aquarium slowly over time, giving the biological filtration time to catch up to the load.
_________________

[webig]

Great article Jan! And a great place to post it!! Good job everyone.
_________________
__________________________________
In Spring, at the end of the day, you should smell like dirt.

[ayoda]

Very nicely done Jan! An excellent overview, it's quite helpful!

[RJSorensen]

I did a quick read of this . . . and it is very good Jan. Your insights in many regards parallel mine and I would be quite happy to place me and my fish in your care.