Nitrogen Cycle

Date Posted: 2022-06-15 17:50:10

The Easy Guide to the Nitrogen Cycle for Aquariums

Are you getting started with your first fish tank?

Then you may have heard of something called the “aquarium nitrogen cycle,” followed by a bunch of complicated scientific terms and graphs that seem a little overwhelming.

No need to panic! Keep reading as we explain the nitrogen cycle in this very short and simple guide.


What is Nitrogen Cycle??

Nitrogen cycle is a very important process for the establishment of beneficial bacteria in the aquarium that will help in the conversion of ammonia (NH3) to nitrite (NO2), nitrite (NO2) to nitrate (NO3).


Nitrogen Cycle Stage

Initial stage

  • Cycle begin when fish are introduced into the tank.
  • All the decompose waste (faeces & uneaten food) are quickly broken down into either ionized ammonium (NH4) or unionized ammonia (NH3).
  • Ammonia (NH3) usually begin rising by the third day after introducing fish.

Ionized Ammonium (NH4):

  • It is present if the pH is below than 7.
  • It is NOT TOXIC to fish.

Unionized ammonia (NH3):

  • It is present if the pH is 7 or above.
  • It is HIGHLY TOXIC to fish.

Second Stage

  • Bacteria called Nitrosomonas Bacteria will develop and they will oxidize the ammonia in the aquarium and eliminating it.

Third Stage

  • The by product of ammonia oxidation is Nitrites (NO2).
  • Nitrites (NO2) are just as toxic as ammonia.
  • Nitrites (NO2) usually begin rising by the end of the first week after introducing fish.


Forth Stage

  • Bacteria called Nitrobacter will develop and convert the Nitrites (NO2) to Nitrates (NO3).
  • Nitrates (NO3) are not highly toxic to fish in low to moderate levels.

Way to maintain the Nitrates (NO3) level

  • Routine partial water changes.
  • Check the water quality by using test kit.
  • For freshwater tank, water plants will help to use up some Nitrates (NO3).
  • For saltwater tank, live rock can break down the Nitrates (NO3).

Effect of excess level of Nitrates (NO3)

  • Growth of algae as Nitrates is source of the food for algae.
  • Increase level of phosphate (PO4).

Benefit of phosphate (PO4)

  • Essential for tissue growth of fish.

Harmful of phosphate (PO4)

  • Create algae bloom and green water.