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How Do You Cycle an Aquarium – The Nitrogen Cycle

If you are new to aquariums then you need to know right away that this is an extremely important topic when it comes to starting a new freshwater aquarium and the continued maintenance of a healthy aquarium. I hear a lot of newbies asking the question how much nitrite will kill your fish or how much nitrite is too much in aquarium without really understanding where the nitrite came from in the first place and what else they should watch out for.


This article will be a high level look at what nitrites are as well as what ammonia and Nitrates are. All of these things are part of the Nitrogen cycle. If you take the time to read this whole article then you will have a very good grasp on this topic so that you will be able to confidently start and maintain a healthy aquarium. After getting started reading this article you will realize quickly that you will need a water testing kit. Make sure you have your kit before you get started.

For a water test kit I highly recommend the API freshwater master test kit. This kit tests everything covered in this article as well as PH levels. I will add a link to Amazon here so you can check it out. Freshwater Test Kit.



What is the Nitrogen Cycle in a Fish Tank?


  • To really understand what Nitrites are and how they happen you really need to understand what the Nitrogen (or biological) cycle is of an aquarium. The most common phrase used and understood by beginner aquarists is cycling your aquarium. This is all about getting your aquarium ready so fish can be added safely.
  • If you don’t understand the Nitrogen cycle or again most commonly known as the cycling your tank phase after first setting up your aquarium you run in to the danger of killing your new fish.
  • So when the employee at the pet store you bought your tank from told you, you have to cycle your aquarium water before you put fish in it this is what he/she was talking about.
  • At about as high level as I can get regarding this topic the Nitrogen Cycle is about ammonia being introduced in the aquarium by a few different ways then bacteria turning the ammonia into Nitrite which then turns into Nitrate. That’s basically it. Once you understand how to start the nitrogen cycle and how to monitor the different levels before you add fish and after you will have success in this hobby.
  • The Nitrogen cycle is when you are wanting to create healthy bacteria in your water, filter and aquarium bed. Which means the Ammonia, Nitrite and Nitrate levels will all be at a safe level for fish to be able to survive in the tank.


What should Ammonia, Nitrite and Nitrate Levels Be in a Freshwater Aquarium?


  • The goal here with this nitrogen or biological cycle is to get your ammonia levels to 0.
    • the Nitrite levels to 0
    • the Nitrates under 40pm but ideally somewhere under 20ppm as even though under 40ppm is safe for your fish it is still not 100% safe.

How Long Does it Take for a Fish Tank to Cycle?

  • Aquariums can take anywhere on average between two weeks to two months to cycle depending on which route you go in cycling the tank.
  • Typically the longer it takes for your aquarium to finish cycling the safer and more stable the water is for your new pet fish.

How to Start the Nitrogen Cycle

First of all let’s take a high level look and cover what happens during this process then we will look at some different options to get the ball rolling for your set up.


  • To kick things off you must somehow introduce ammonia to the tank.


  • Once this is done in a short amount of time bacteria will appear and turn the ammonia into Nitrites.



  • Then through small water changes and the filtration system running along with developing bacteria the Nitrites over time are converted to Nitrates. Nitrates are not as toxic to fish and can be tolerated at low doses.


  • Nitrite and Ammonia are toxic to the tank and will kill inhabitants if the levels of these compounds get out of hand. As soon as the levels rise above zero your fish will start feeling the effects. Depending on how hardy and tolerant the fish are will dictate how quickly they start to get sick and eventually die. Don’t let that happen.


  • These biological levels should be monitored on a regular basis to ensure the process is going the way it should be. Test every 2 to 3 days. I know some users suggest every week however I want to know as quickly as possibly if I should be adjusting anything to make sure everything runs as smoothly and quickly as possible so I can buy some fish.


  • Again, all of this can take anywhere from 2 to 8 weeks on average. It could take longer but that is the average.


Options for Starting the Nitrogen Cycle


Option #1 Adding live Bacteria

What I would consider the most effective option is to take the filter medium from an established aquarium and place it in your filter. My golden tip if you choose to go this route is to talk to someone at your local pet store and see if you can possibly get something from one of their tanks whether it be some filter medium they are just changing or a big scoop of the gravel. This will surely get your aquarium firing on all cylinders as quickly as possible.

  • You can also take some gravel from the established aquarium as active healthy bacteria should be abundant in the gravel and add that to your new aquarium setup.
  • This way you are introducing healthy bacteria right away.
  • It also helps to add fish flakes to the tank when you go this route. By doing these things you might be able to see healthy Nitrite, Ammonia and Nitrate levels in as little as less than a week.
  • If you have the luxury of adding a new filter onto an existing established aquarium a couple of weeks before you fill up your new tank you could then just take that new filter and move it over to the new tank.
  • Or you could possibly get a friend to put your new filter on their existing tank so you could have some nice healthy bacteria started by the time you are ready to kick start the Nitrogen cycle.
  • So there is some risk using this option however the turnaround from new set up to cycled and ready for fish is so fast that maybe it is worth the risk if you feel you can trust the health of the established tanks filter medium.
  • For saltwater tanks a great medium to introduce is live rock which will be teaming with live bacteria ready to start working on the cycling of your water. Live rock is expensive however well worth it and most likely if you are doing a saltwater aquarium you are already aware of the high costs in the saltwater hobby versus the tropical fish side of the hobby.
  • The different stages of the cycle will process much quicker making the time you first start your filtration system to putting fish in the tank much quicker than most other options.
  • Some things to consider if you go this route:
    • If you take filter medium from another aquarium you are taking any bacterial issues from that tank and putting it into your new set up.
    • If there are algae blooms attached to the filter you are adding algae to your tank.
    • If there are any parasites than can harm fish in the established tank you are potentially adding those as well.
    • Make sure to transfer the medium ASAP to the new filter body as the living bacteria will die off very quickly.


Option #2 Adding Live Fish

  • Adding fish to a new aquarium is an option however not a good option in my opinion.
  • Fish excrete urea containing ammonia. The ammonia breaks down with the help of bacteria which then converts into Nitrite.
  • When adding fish you are using these living creatures to start the Nitrogen cycling process off by having the fish poop in the water. Yes this triggers the biological process.
  • There will also be uneaten food in the tank that will start this process as well.
  • Many experienced aquarium owners recommend using tiger barbs for establishing a tank as they are an extremely hardy fish. If you have no interest in keeping barbs as pets then I would suggest you stay away from this option or you could try another hardy fish that you are interested in keeping.

Option #3 Adding fish food or shrimp.

  • If you are adding fish flakes just add a few flakes at a time. Since there are no fish to eat any of the food you do not need to add very much. Same rule goes with adding a bit of brine shrimp each day.
  • Some experienced aquarists will recommend not using fish flakes as some brands contain phosphate which will end up sparking the growth of algae in the aquarium. Even though some bottom feeders and fish might like to eat algae it is very much a nuisance and can be terrible to clean if it gets out of control.
  • By adding fish food to the aquarium you are starting the nitrogen cycle when the food decomposes it releases ammonia kicking off the cycle.
  • Make sure to keep adding food once a day at minimum. You can add food a couple times of day.
  • If you add raw fish or shrimp try adding a chunk about an inch by an inch and monitor ammonia levels in the tank after a couple days. If you find the ammonia is not rising yet try adding another small piece. I would rather add small pieces at a time and have just enough added than throw a huge piece of raw fish in the tank and have a big mess.
  • I feel that a larger piece of fish will also extend the whole Nitrogen process and you want to start adding fish as soon as possible so not a good idea.

Option #4 – The Ammonia Cycle

  • By not adding fish or food you can add ammonia to kick start the Nitrogen cycle. By not having fish in the aquarium you can increase the amount of ammonia which should speed up the process
  • Using 100% pure ammonia is a great way to start this process.
  • Using ammonia instead of fish or fish food is a better way of doing this yet most people shy away from this one.
  • I really like this process if you are a patient person. After this process is complete and ready for fish you tank will look squeaky clean. Much nicer than the other options in this aspect.
  • Make sure the ammonia you pour into the aquarium only contains ammonia and water. If there are other elements in the ammonia it could cause other issues with your water.

How much ammonia should you put in aquarium

  • I would recommend putting approximately a ¼ teaspoon of 100% ammonia in per day for a few days and testing after a couple days to see if you can get a reading of ammonia.
  • If you are not getting a reading after a few tests or find the ammonia levels are not changing feel free to add more ammonia each day.
  • You want to see a spike in the Ammonia level and then see it start to drop.
  • Once you get a Nitrite reading stop adding the ammonia so the Nitrogen process can proceed naturally after that.

How to test biological levels in an aquarium

  • Plain and simple there are test kits available that will check the levels on Nitrites, Ammonia, Nitrates and more.
  • You can test PH and hardness as well.

Chemicals for controlling Ammonia, Nitrites and Nitrates

  • By adding chemicals to your tank you can help the Nitrogen cycle move along.
  • These products can bind Nitrites and Ammonia and actually detoxify them. One product that stands out is called Prime
  • Tetra safe start is another excellent product it is a live bacteria product
  • Bio-Spira for nitrogen cycle speeds up process ( saltwater aquariums)

What kind of water should you use in an aquarium?

  • If you use tap water for aquarium make sure to add a chemical to the water that will nullify any chlorine, Or alternatively you could pour tap water into a large pail and let it sit for a few days before putting into the tank. This is what I do. I am always refilling the pail after small water changes. It is cheaper than purchasing an anti-chlorine product.
  • some hobbyists will suggest you use distilled or reverse osmosis water but i like using good old tap water and just letting it sit for days before it gets thrown into the aquarium.

When is the Nitrogen cycle complete?

  • When the levels of Ammonia are lowered to 0.
  • When the levels of Nitrites are lowered to 0.
  • When the Nitrate levels are lower than 40ppm but ideally under 20ppm.
  • This is when your tank is cycled and ready for fish.
  • Even when the tank is at the safe biological levels introducing to many fish at one time will cause a spike in Ammonia and then Nitrites all over again.
  • Make sure to only add a couple of fish at a time and in weekly intervals.
  • This way your aquariums bacteria will have time to filter and process the little bits of Ammonia Nitrites and Nitrates  without disturbing the health of the tanks inhabitants.
  • Something else to consider is that smaller aquariums somewhere around 10 gallons and under will have more fluctuations in these components levels than a larger tank which will be more forgiving.


Bonus Tips Just For You

  • For a water testing kit I would recommend the API FRESHWATER MASTER TEST KIT as mentioned earlier in this article. There are other great test kits on the market as well. This one has a great track record and tons of customers. Check it out on Amazon API TEST KIT.


  • It is very important that when you are cycling that you do not clean up extra food or anything else you see in the tank before the process is complete. If you clean while the aquarium is going through the Nitrogen cycle you risk levels of Ammonia and Nitrite shooting up again to unsafe levels. Just wait until everything is ready and then you can do a small cleaning. Even at that point I think you should only do small bits of cleaning here and there especially if you are performing partial water changes.


  • Once you feel your Nitrogen cycle is complete you can start adding a couple of fish. Do not overdue it and add to many fish as that will cause a spike again in the Ammonia and Nitrite levels basically kicking off the Nitrogen Cycle again.


  • Some aquarists believe that if you increase the aquariums temperature to somewhere around 80 degrees that it will speed up the time it takes for your tank to cycle. If you don’t have fish in the tank or have fish that are okay with that temperature then give it a try if you are in a hurry.


  • To lower the nitrate levels below 40ppm weekly partial water changes will help you get to this level. Perform small partial water changes keeping track of how much water taken out as well as what your levels are in the tank previous to the change and a day later to see how the change affected the tank. Adjust the amount of water taken out based on how the levels measure the day after the change compared to the measurement before the change.


  • When done and fish are added do not clean your filter medium with tap water. This will kill all of the healthy bacteria in the filter medium. This will also cause the Ammonia and Nitrites to spike a bit in the water afterwards. Use some water out of the aquarium or you can have a large pail of water that was set out a few days earlier. I prefer using water right from the aquarium. Some tank owners think that they have to clean the filter foam should be squeaky clean like when it was taken out of the package however that is not the case. Running a healthy tank is what is most important.


So there you have it. There is a lot of content in this article yet it all boils down to the fact that to have a healthy tank so you can start adding fish to it you need to get the Ammonia, Nitrite and Nitrate levels to a safe level so fish added will live long happy and healthy lives. After reading this I am sure you agree that this process is really quite easy. It is just about measuring the biological levels and adjusting for success.

Related Aquariums-at-Home Articles.

How Often Should You Clean Your Aquarium Filter?

How To Get Rid Of Algae. 

What are the Signs of Ammonia Stress in a Fish Tank?

How to Test Fish Tank Water Without a Kit

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