Aquarium Fish

Aquarium Nitrogen Cycle

Updated July 10, 2018 | Mike FishLore

The aquarium nitrogen cycle information presented below may be rather boring to most people, but it is absolutely essential to understand this process if you want to be successful at keeping fish!

Nitrogen Cycle Overview

Steps in the Process:

  1. Fish Waste & other biological processes

  2. Ammonia

  3. Nitrites

  4. Nitrates

  5. Water changes to remove nitrates and DOC

Some call it the biological cycle, the nitrification process, new tank syndrome or even the start-up cycle. They all are referring to the same cycle - the nitrogen cycle. The aquarium nitrogen cycle is a very important process for the establishment of beneficial bacteria in the aquarium and in the filter media that will help in the conversion of ammonia to nitrite and then the conversion of nitrite to nitrates. This process can take from 2 weeks to 2 months or longer to complete. It is vital for anyone planning on keeping aquarium fish to learn and understand this process.

The best way to monitor the nitrogen cycle is to purchase an aquarium test kit that will test for ammonia, nitrites, nitrates and ph. While you certainly can get the individiual test kits for ammonia, nitrite, nitrate and pH... the cheapest option is to get a master test kit that contains all of these tests in one kit. See below:


Get this test kit or one similar

Test your aquarium water every other day and write down your test readings. You will first see ammonia levels rising. A few weeks or so later you should see the nitrite levels rising and the ammonia levels dropping. Finally, after a few more weeks you should see the nitrate levels rising and the nitrite levels dropping. When you no longer detect ammonia or nitrites but you can detect nitrates you can assume that it is safe to add fish to your aquarium.

Aquarium Nitrogen Cycle
Photo Credit: Ilmari Karonen

Nitrogen Cycle Stages
Stage 1
Ammonia is introduced into the aquarium via tropical fish waste and uneaten food. The tropical fish waste and excess food will break down into either ionized ammonium (NH4) or un-ionized ammonia (NH3). Ammonium is not harmful to tropical fish but ammonia is. Whether the material turns into ammonium or ammonia depends on the ph level of the water. If the ph is under 7, you will have ammonium. If the ph is 7 or higher you will have ammonia.

Stage 2
Soon, bacteria called nitrosomonas will develop and they will oxidize the ammonia in the tank, essentially eliminating it. The byproduct of ammonia oxidation is Nitrites. So we no longer have ammonia in the tank, but we now have another toxin to deal with - Nitrites. Nitrites are just as toxic to tropical fish as ammonia. If you have a test kit, you should be able to see the nitrite levels rise around the end of the first or second week.

Stage 3
Bacteria called nitrobacter will develop and they will convert the nitrites into nitrates. Nitrates are not as harmful to tropical fish as ammonia or nitrites, but nitrate is still harmful in large amounts. The quickest way to rid your aquarium of nitrates is to perform partial water changes. Once your tank is established you will need to monitor your tank water for high nitrate levels and perform partial water changes as necessary.

There are other methods to control nitrates in aquariums besides water changes. For freshwater fish tanks, live aquarium plants will use up some of the nitrates. There are now products you can run in your filter to remove nitrates. One such product is made by Seachem and is called Denitrate. For saltwater fish tanks, live rock and deep sand beds can have anaerobic areas where denitrifying bacteria can breakdown nitrates into harmless nitrogen gas that escapes through the water surface of the aquarium. While nitrate is not as harmful as ammonia and nitrite, you still don't won't it getting too high and you need to figure out your plan for keeping it low for long term success (e.g. water changes, live plants, live rock, filter supplements, etc).

Now that we have a good understanding of what takes place biologically in the aquarium, lets...

Go To Part 2 - How To Start The Aquarium Cycle


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