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The Importance of the Nitrogen Cycle in a Freshwater Aquarium

Updated on April 23, 2014

When establishing a new aquarium, it is important to create a biologically stable environment for the fish. The nitrogen cycle is the process of establishing beneficial (or good) bacteria that eliminate ammonia and nitrites in the water that are toxic to fish. The cycle begins after fish are introduced to the aquarium and can take about four weeks to complete. Failure to establish beneficial bacteria can result in a condition called "new tank syndrome," which can kill fish. The nitrogen cycle has three stages: (1) ammonia is introduced; (2) nitrosamonas oxidize and eliminate ammonia, creating nitrites; and (3) nitrobacter converts nitrites into less harmful nitrates.

This is a male Apistogramma panduro, which is a dwarf cichlid from Peru.
This is a male Apistogramma panduro, which is a dwarf cichlid from Peru. | Source

Stage 1: Build-up of Ammonia

The first stage of the nitrogen cycle starts with the introduction of ammonia. Ammonia is a toxic substance that can build up in an aquarium through the decay of nitrogen products such as fish waste and uneaten food. After fish are introduced to the aquarium, all of their waste and uneaten food is broken down into either ionized or un-ionized ammonia, depending on the acidity (pH) of the water. The ionized form, ammonium (NH4), is not toxic to fish and is present when the pH level is below 7. The un-ionized form, ammonia (NH3), is toxic to fish and is present when the pH level is above 7. Ammonia build-up usually begins after the third day of introducing fish to the aquarium.

Stage 2: Nitrosamonas Produce Nitrites

The second stage of the nitrogen cycle detoxifies ammonia into nitrites, which are less harmful to fish. Nitrosamonas are good bacteria that grow in an aquarium with a good oxygen supply (through filtration and airstones). Nitrosomonas oxidize the ammonia in the aquarium, which eliminate it, but produce nitrites. A nitrite is not as lethal as ammonia, but it is still a highly toxic chemical that can cause stress to fish at 0.5 parts per million (ppm) and cause death at 10 to 20 ppm. Nitrite levels usually begin rising after the first week of introducing fish to the aquarium.

Stage 3: Nitrobacter Converts Nitrites

The third stage of the nitrogen cycle converts nitrites into another less harmful compound called nitrates. Nitrobacter is another good bacterium that grows as ammonia levels decrease. Nitrobacter consumes the nitrites in the aquarium and converts them into nitrates. While nitrates at low to moderate levels are not harmful to fish, high levels can be lethal. To prevent high levels of nitrates in an aquarium, test nitrate levels using an aquarium testing kit every three months and routinely change the water. As nitrates increase and nitrites decrease, often small green algae will bloom. The green algae as well as improved appetite and improved behavior in the fish indicate the nitrogen cycle is complete.

The Nitrogen Cycle

This image summarizes the nitrogen cycle for an aquarium.
This image summarizes the nitrogen cycle for an aquarium. | Source

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New Tank Syndrome

New tank syndrome occurs when there are dangerous levels of ammonia and nitrites that are not converted to nitrates. A common cause of new tank syndrome is overstocking fish (adding too many fish) when setting up an aquarium or adding more fish before the nitrogen cycle is complete. Before adding more fish to an aquarium, make sure ammonia and nitrite levels are at zero. This prevents the new fish from being stressed by poor water conditions, conditions that the original fish have become accustomed. New tank syndrome can be difficult to detect and can kill fish in little time. If fish exhibit symptoms such as color loss, clamped fin, or lying near the bottom of the aquarium, test the ammonia and nitrite levels and change water as needed.

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    • shaney williams profile image

      shaney williams 3 years ago

      i like how u write it s very good

    • Lacey Taplin profile image
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      Lacey Taplin 3 years ago from Highlands Ranch, CO

      Thank you, Shaney!

    • one2get2no profile image

      Philip Cooper 3 years ago from Olney

      Great hub...voted up.

    • Lacey Taplin profile image
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      Lacey Taplin 3 years ago from Highlands Ranch, CO

      Thanks for the vote!

    • prasetio30 profile image

      prasetio30 3 years ago from malang-indonesia

      Very informative hub. Thanks for sharing and I love your tips. Voted up!

    • Lacey Taplin profile image
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      Lacey Taplin 3 years ago from Highlands Ranch, CO

      Thanks for the vote, prasetio30!

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