The Nitrogen Cycle - To Make Your Tank Fish Safe

  • Jun 10, 2021
  • Rita
  •   20        0      0

Are you setting up a new fish tank or did you just discover aquarium keeping as a new hobby? Keeping fish can be an amazing experience, but it’s not as simple as buying the tank just to put in the fish the next day. You might have had people telling you that you will need to cycle your tank before you get fish but what does that really mean? I’m here to help you to understand what happens when a tank get’s cycled and will help you to do it properly. 

The Nitrogen Cycle - What Is It?

In nature, the nitrogen cycle is the process where nitrogen compounds get broken down into harmless substances that can be used as plant food. Initially, nitrogen moves from the air to the soil due to some chemical reaction in the air. There it gets absorbed by plants and enters the food chain. When the plants are eaten by animals the nitrogen is excreted in the feces and other excrements of animals. These compounds can be toxic to living beings like fish. That is where the bacteria comes in. The bacteria break down harmful compounds like ammonia and turn it into a form that plants can absorb again.

In the wild, this cycle happens naturally without human intervention, but that is not true for your aquarium… at least initially. When you set up a new aquarium, the tank and everything that goes in it are sterile or close to sterile. This means that there is no bacteria present that can break down ammonia from fish feces into nitrate and nitrite that can be used by the plants. The process of developing the bacteria is what we refer to as cycling the tank.  

How To Cycle Your Tank

Cycling your tank basically means that you’re establishing an ecosystem that removes waste products. For fish to be able to survive and thrive in your tank, the waste removal system must already be in place to deal with their feces. If not, the toxic compounds excreted by fish like ammonia, nitrates, and nitrates will quickly build up in your tank and poison your fish without your intervention. 

Cycling your tank is really quite simple. All you have to do is set up your tank, design it as you please fill it with water, and start your filter. If you have another tank, you can buy a filter before you get your new tank and set it up in your functioning tank to start the cycling process. The filter media will pick up bacteria from your existing tank which makes cycling your new tank much faster.

If this is your first tank, don’t be disheartened. The cycling will take a bit longer, but the bacteria will definitely still develop in your tank. You’ll just have to wait a bit longer before you get any fish. 

It is possible to force your tank to cycle faster by introducing some fish early on. Doing this is risky, however, since your fish may be poisoned if you don’t pay close attention to water parameters. Introducing fish will force the bacteria to develop faster since the ammonia levels in the tank will rise rapidly.

To keep your fish safe and alive during this time, you will have to water changes every day. Usually, it takes a tank around one month to develop the beneficial bacteria, but with a fish, it can take anywhere from two weeks to a month. You’ll know your tank is ready for more fish when the ammonia and nitrate levels in your tank are none existent even though a fish is present. It is always a good idea to stock your tank with smaller, younger fish slowly over time to allow your bacteria to develop and keep up with the waste being produced. Also, make sure that you never overstock your tank since this will be detrimental to your fish no matter what you do. 

Ammonia Spikes

Ammonia spikes happen when your tank doesn’t have enough beneficial bacteria to break down this harmful substance. You will usually see an ammonia spike if you first introduce fish to your tank before it is cycled, your fish are being overfed, or if your tank is overstocked. You can lower the ammonia level in your tank by allowing it to cycle before you introduce fish or by doing frequent water changes to remove decomposing fish waste. In a healthy tank, the beneficial bacteria also called nitrogen-fixing bacteria, will convert ammonia into nitrites. This is step one of the biological filtration system in your tank. 

The bacteria will usually multiply very quickly to catch up and deal with the excess of ammonia in your tank. This phenomenon can be observed. Your tank will suddenly appear murky with white, smoke-like substances floating around. Don’t panic, your tank will be back to normal in a few days. 

Nitrite Spike

Nitrites are the most dangerous of the nitrogen compounds produced in your tank. They are the usual culprits when it comes to unexplained fish deaths. Nitrites will usually also spike after you had an ammonia spike since they are produced by the bacteria that break down ammonia. As a result, you will have another bacterial bloom as the nitrite-loving bacteria multiple to keep up with the excess amount of nutrients suddenly available to them. 

Nitrites are converted into nitrates by nitrite-loving bacteria, effectively rendering them harmless. If you want to prevent a nitrite spike, make sure to feed your fish sparingly.  


Nitrate is the product you end up with after the while nitrogen cycle has been completed. In small doses, nitrates are not harmful to fish, but they can cause other problems like algae blooms. You can control the amount of nitrates in your tank by doing regular partial water changes and planting more live plants in your tank. Plants use nitrates to grow and reproduce and will also release oxygen into your tank water. 

Once your tank is properly cycled, you can finally introduce some fish. To make sure you don’t get any spikes, introduce fish in small numbers. Keep an eye on your water parameters by using a testing kit you can buy from most pet shops. If you see any spikes, do partial water changes to protect your fish. Once it’s all done and settled, you can finally sit back, relax and enjoy your new aquarium. 


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