Science of Freshwater Aquariums (part II)

After discussing starter equipment, we will discuss nitrogen cycle - most important segment of aquarium ecosystem.

Nitrogen Cycle

Nitrogen cycle

Nitrogen Cycle begins with decomposing of organic matter. This includes uneaten food, plant parts and fish feces. When it decomposes it results in ammonia which is highly toxic to fish in aquarium. Smaller part of ammonia is used by plants and greater part is used by Nitrosomonas (Nitrosomas) to produce nitrites. Since Nitrites are still highly toxic it needs to be converted yet again. Nitrospira converts dangerous nitrites into less toxic nitrates. Nitrates are used by plants and are removed from aquarium by water changes.

Step-by.step cycle

Aquarium Cycling

When you set up your new aquarium it is required to cycle it. Cycling means allowing time for bacteria colonies to form in aquarium which help dissolve organic waste into less toxic compounds. This process is usually done without any animal presence (because of high toxicity), however some fish keepers cycle with few fishes that are more resistant to ammonia/nitrites/nitrates.

These are the steps you should take to cycle your aquarium:

1) Ammonia - To start the cycling process first you need ammonia. You can "produce" some by putting some fish food in aquarium and let it decompose.

2) Oxygen - Since Nitrosomonas are aerobic bacteria, they require as much oxygen as possible. This can be done by letting the water pump agitate surface of water and by adding oxygen stones in aquarium.

3) Light - These bacteria love light and if there are no fish in aquarium, light should be kept on all the time.

4) Temperature - Higher temperatures allow bacteria to grow even faster. If there are no fish in aquarium temperature should be kept around 28C (83F).

5) Filter - In order to accumulate bacteria in filter material, filter has to be running all the time.

6) Plants - Plants reduce nitrate and ammonia levels and produce oxygen for Nitrosomonas.

7) Starters - There are bacterial colony starters available in liquid form. These are usually dropped on filter to make starter colonies.


After cycling has started, three phases can be expected:

Cycling of a new tank

1) Ammonia spike - after few days Nitrosomonas will start appearing. This is so called ammonia spike and from that point on, ammonia levels will go down and nitrite levels will go up. Shortly afterwards Nitrospira will appear and start producing nitrate.

2) Nitrite spike - when ammonia levels hit 0, nitrite level will spike. At this point nitrate level will start going down and nitrate level will continue to grow even faster.

3) Nitrite 0 - at one point nitrate levels will go down to 0. This shows that the cycle is complete. Due to high nitrate level around 40% of water should be changed.

After few more days of measuring ammonia/nitrite/nitrate levels and making sure ammonia and nitrite are 0, it is safe to introduce fish into aquarium.

Oxygen Cycle

Finally there is oxygen cycle. This is a very small part of ecosystem balance. Fish need oxygen (O2) dissolved in water in order to survive. This is accomplished by agitation of water surface (with pump), oxygen stones and plant oxygen production (photosynthesis). Animals and plants also produce carbon dioxide (CO2). This is reused by plants for photosynthesis or it exits ecosystem via surface agitation.

Keep in mind:

• more animals means less oxygen and more carbon dioxide
• less light in planted aquarium means less oxygen and more carbon dioxide

Bear in mind that some fish need to go to surface and take oxygen (Anabantoidei - Labyrinth fish) like bettas and gouramis, so it is wise to keep the surface area accessible for those fish.