April 18

Nitrogen Cycle in Aquarium – Easy Steps for Beginners


nitrogen cycle

As a new aquarium owner, it is vital for you to know the significance of cycling your fish tank so that you can keep healthy livestock.

The filtration process in fish tanks can be chemical, mechanical, or biological. However, for a sustainable aquarium, healthy plant growth and effective biological filtration are essential. Tropical fishes, while in their natural habitats may not regularly be exposed or subjected to high nitrogen levels that are toxic to their health, but the situation requires careful and close monitoring in the fish tank where nitrogen pollution, due to overfeeding and some other conditions, may cause your pet to be unwell and even die if the case is not adequately addressed.

Let’s have a glance at the essential compounds of a Nitrogen Cycle

  • Ammonia

In an aquarium, ammonia build-up is often a result of having too many fishes together or feeding them more often. However, ammonia is only poisonous to fish if it is present in high concentrations. In a well-balanced aquarium, the nitrogen-fixing bacteria can oxidize the ammonia and convert it from nitrite to nitrate to make it harmless.

  • Nitrite

Nitrites, which are the salts of nitrous acids, are considered most lethal to the fish in the aquarium. Nitrites are generated from the reduction of nitrates or oxidation of ammonia. Any concentration of nitrites in the water can be poisonous to the fish.

Nitrite formation is more common in fish tanks that are overpopulated as such aquariums use more oxygen. Decaying fish, snails, plants and leftover food have also been found to contribute to the formation of this dangerous compound.

The ideal way to make sure that there is no nitrite build-up in an aquarium is by feeding the fish in lesser quantity and not housing too many pets in one tank. Also, a regular water change, which doesn’t go beyond 20% of the total water volume, can prevent nitrite formation.

  • Nitrate

Nitrate is the end-product of the nitrogen compounds which occur through the breakdown of ammonium compounds such as leftover food, urine, fish excrete, and decaying plants in the tank. The angelfish in both aquarium and natural habitat are tolerant of even large concentrations of nitrate in the water. But, it is advisable to avoid large quantities of nitrates by feeding the pets sparingly and housing a small population of fish that can easily adjust to the size of the tank.

What is the Nitrogen Cycle?

The nitrogen cycle comprises the process through which the nitrogenous compounds including ammonia, nitrite, and nitrate get circulated to process the waste in naturally occurring water. However, in a closed tank, the nitrogen cycle has to be manually established to maintain healthy livestock.

Creating the nitrogen cycle in a new fish tank can take anywhere up to 90 days. It is often termed as nitrification process and new tank syndrome as well. To establish beneficial bacteria in the fish tank and filter system, the nitrogen cycle is critical.

As ammonia, nitrite, and nitrate are common toxic compounds in a fish tank, it is crucial that a nitrogen cycle works appropriately so that these pollutants can be controlled.

If you are setting up a new tank, then it is highly recommended to house the aquarium with young fish in a smaller population so that the nitrogen cycle can be established eventually along with the gradual increase of waste compounds in the water. You need to carefully decide how you can cycle your aquarium to ensure the longevity of your fish.

The ideal way to monitor the nitrogen cycle in an aquarium is by getting an aquarium test kit that can test the levels of different compounds and pH. While setting up an aquarium, you can either buy individual test kits of various toxic compounds or purchase a master kit that can give you an estimate of all the compounds at once.

In order to set up an aquarium for the first time and establish the nitrogen cycle, you need to test the water in the tank daily until you see the levels of ammonia increase. As time progresses, you will observe a rise in the levels of nitrite, and ultimately, nitrate levels will hike up. Once you can see that there is no ammonia or nitrite – which should happen as the nitrogen cycle is established – in the water, you can consider it safe to add your fish to the tank finally.

To demonstrate how the nitrogen cycle correctly gets established, here is detailed information on the various stages in the nitrogen cycle:

    • STAGE I

    When ammonia is introduced into the aquarium water from fish waste, rotten plants, and leftover foods, it can be broken down or can exist in two forms – un-ionized ammonia and ionized ammonia. Ammonia is known to be harmful to fishes and other aquatic pets while ammonium is not.

    Water pH also plays a critical role in the generation/abundance of these compounds as a value in excess of 7.0 will lead to the generation of ammonia while one below 7.0 results in the generation of ammonium. It is important to note that leaving the pH constantly at a value above 7.0 and failure to cycle the aquarium could eventually lead to the accumulation of ammonia to a hazardous level.

    This can be resolved by introducing healthy bacteria in the tank. When present in a fish tank, these bacteria keep growing in number and consume ammonia as soon as it appears. You will get to know whether these healthy bacteria are there in your fish tank when the level of ammonia drops after a week. Thereafter, the nitrogen cycle enters the next stage.

    • STAGE II

    When the bacteria oxidize all the ammonia in the aquarium, it leads to the creation of nitrites as the by-product.

    Nitrites can be extremely harmful to your fishes and need to be eliminated immediately. You can quickly test the level of nitrites in your tank by the end of the second week of using a test kit. But similar to the first stage, here also the good bacteria appear in the aquarium and consume the nitrites as quickly as they are created. Eventually, you will observe a decline in the level of nitrites in the tank, which means that the water has entered the last stage of the nitrogen cycle.


    Once the level of nitrites starts declining in the water, you will see a rise in the level of nitrates. This is because of the activities of the bacteria called Nitrobacter, which oxidize nitrites into nitrates – nitrogenous compounds that are relatively less harmful. These nitrates are the final product in the nitrogen cycle. However, if nitrates keep building up, they can cause harm to the fishes.

    The easiest way to eliminate nitrates from your fish tank is by performing a water change regularly. After setting up your aquarium, you will need to monitor the level of nitrates often and perform water change accordingly.

    Alternatively, if you have a fresh water tank, then using live plants can help in reducing the nitrate levels as they are capable of using up the compound. Also, you can certain products with your filters to get rid of nitrates. If you have a saltwater aquarium, then deep sand beds which have bacteria to break down the nitrates to nitrogen can be helpful.

    That is all! Now that you have understood what exactly goes on with a nitrogen cycle, you should be ready to set up a new aquarium. Cycling a fish tank implies that you are making your aquarium undergo a nitrogen cycle manually. Even if you already own a fish tank, it is essential to cycling it so that the healthy bacteria can grow to a certain point where they can oxidize the lethal levels of ammonia and nitrites as soon as they are produced.

    If you are leaving your fish in a tank that is not adequately cycled, then remember that the nitrogen cycle is a natural process and it will kick-start as soon as any waste is introduced on the water. However, when you are not monitoring this cycle, and there are high concentrations of ammonia and nitrites in the water, then this puts your fish at high risk of getting sick and ultimately dying. That is why, if you think that the tank can cycle on its own, then you are making your pet more susceptible to diseases.

    How to do a Fishless Aquarium Cycling

    Performing a cycle in a tank without risking the lives of the fishes is highly recommended. There are several methods to do a fish-less cycle, and we shall now share the best way to do this in a step-by-step tutorial for you.

    Before you begin to cycle an aquarium, first you have to purchase an aquarium test kit, so that you can find out what is happening inside your fish tank.

    If you follow these instructions carefully, you will find cycling your aquarium pretty easy.

    Step 1: Prepare the fish tank

    You need to start with a fully assembled 5-gallon fish tank which includes an air pump, heater, filter, plants, etc. Note that it is essential to have a complete set-up in place so that the beneficial bacteria can have a surface to reside on. Furthermore, ensure that you turn on the tank’s heater and filter throughout the cycling process. Again, it is highly important to note that good bacteria in an aquarium survive at a temperature of 65-85 degrees Fahrenheit. Also, it is imperative to check the pH level of the water in your fish tank as any pH below seven can stop the nitrogen cycle completely.

    Step 2: Introduce ammonia in the tank

    Now you need to create an optimal environment where ammonia can be produced. As there are no fish in the tank, you will need to introduce ammonia at this time manually. To do so, you have three options:

    • Sprinkle some flakes in the tank every twelve hours so that when the food decomposes, it will release ammonia. This process needs a lot of patience as you have to keep putting flakes into the tank for a minimum of a week.
    • Drop a chunk of raw shrimp or fish into the tank, which can decompose and release ammonia
    • Add 100% pure ammonia so that you don’t need to wait for the waste to break down into ammonia. Ammonia can be added to the water in a ratio of 1:2; for instance, 2.5 drops of ammonia can be added to 5 gallons of water. However, you may have to keep adding the ammonia to the water should the test kit not give any trace (of ammonia) upon taking the reading.

    Step 3: Testing for Ammonia

    Test for ammonia every day and maintain the levels at 3 ppm. FYI, ammonia levels higher than 5ppm can stop the nitrogen cycle. At this stage, the healthy bacteria will grow and start oxidizing ammonia. Follow this process for a week to check if the ammonia levels are dropping, and after this, you can move on to the next step.

    Step 4: Testing for Nitrites

    After a week, it is time to check for the nitrite level in the tank. If you see nitrites present in the water, it means that the nitrogen cycle has begun.

    Here, you need to remember that the bacteria are feeding on ammonia and the only source of that compound is provided by you. This means that if you stop feeding ammonia to the tank, then the bacteria will die, and the cycle will eventually stop. So, you need to continue adding ammonia as you were doing before and make sure that the levels remain under 5 ppm.

    Keep monitoring the nitrite level every day, and once you observe that the levels are dropping, then you can move to the final step.

    Step 5: The Final Test: Nitrates

    As you keep feeding the bacteria in your fish tank with ammonia, the level of nitrites will increase to an extent and eventually drop when the good bacteria oxidize it into nitrates. This implies the completion of the nitrogen cycle.

    As the level of ammonia and nitrite reach zero, you need to reconfirm once by adding the same amount of ammonia that you had introduced right from day one. Check the reading after 24 hours, and if the level of ammonia and nitrite still come out as zero, then your tank is fully cycled.

    How to Speed Up the Cycling Process

    If you think waiting for weeks can be frustrating, then you have a few options to speed up the process of the nitrogen cycle in your aquarium.

    • Introducing living plants in the tank can speed up the nitrogen cycle as these plants not only carry beneficial bacteria but also directly pull the ammonia from water for the process of photosynthesis.
    • Increase the water temperature to 27°C-28°C
    • Get some substrate from an established tank that can help transplant good bacteria to the newly set aquarium.
    • Add filter media from another aquarium as it will already have nitrifying bacteria attached which makes your aquarium cycle faster.

    How to Solve Common Aquarium Cycling Problems 

    It is common to experience some very usual problems while you are cycling your aquarium. Here, we have listed a few such issues and how you can get rid of them.

    • Ammonia Stress

    Ammonia poisoning is always a common risk when you are cycling your tank. Look out for the following symptoms to find out if there is ammonia poisoning in your aquarium.

    • Lack of movement
    • Dwelling in the bottom of the tank
    • Loss of appetite
    • Swollen eyes and anus

    If you observe any of these symptoms with your fish, then you have to lower the level of ammonia by changing the water more frequently. If these symptoms are not dealt with immediately, they can result in your fish dying.

    • Ammonia level doesn’t drop

    If the level of ammonia doesn’t drop, then the only possible reason is that the pH of your water is too low. If the pH of the water is less than 7, then the ammonia will be present in the form of ammonium.  Ammonium cannot be oxidized by bacteria. This can also happen if you are using chlorinated water or cleaning your tank more often. As the bacteria which live in the substrate, media, and decorations, cleansing too much can remove these bacteria from the tank.

    • The nitrogen cycle is not starting

    It usually takes a couple of days for the ammonia level to rise in an aquarium. However, if by day 5, you do not see any hike in the level of ammonia, then your nitrogen cycle hasn’t started yet. To resolve this, you need to add more ammonia to the water and remove any living plants if present.

    • Wrapping up:

    Once you have successfully established the nitrogen cycle, you should remember that it never stops. It will keep on running consistently, and you need to regularly monitor the levels of ammonia and nitrite in your tank to make sure that nothing goes wrong.

    How do Detritivores Help in the Aquarium Nitrogen Cycle?

    There are quite a number of organisms that make up the aquatic ecosystem – the aquarium to be precise in the context of this article. While some aquarists may be overwhelmed by the ecstasy of seeing their favorite pet(s) swim across the tank, there are some living organisms that should not be left out of the equation as they make life thrive within the aquarium; this is where detritivores get a mention.

    So, what are detritivores? – Simply put, detritivores are heterotrophs, and are so named for their feeding inclination; they feed on dead organic matter – otherwise known as detritus – that are available within their habitat. Some common examples of detritivores include slugs, millipedes, earthworms, sea cucumbers, woodlice, crabs, springtails, etc.

    Beyond the fundamental fact that detritivores rid the tank of decomposing organic matter, there is a significant role they play in the nitrogen cycle process which may not be very obvious to many persons, especially, aquarium hobbyists, and this has to do with their importance in ensuring that nitrogen cycle flows through easily within the aquarium.

    How you may ask? Before I proceed to answer this question; let me be quick to say that, though there are several detritivores, it is not all of them that can fit into the aquarium – ghost shrimps, slugs and bristle worms could however suffice in this regard.

    Now to the question; it is worth noting that the presence of decaying organic matter in the aquarium leads to the generation of ammonia which could be injurious to the fish at a high concentration when its (ammonia’s) primary consumers – bacteria known as Nitrosomonas – are not available or well-supplied within the tank. This could however be curtailed if the tank having a rich fauna of detritivores.

    More specifically, the availability of detritivores within the aquarium goes a long way in ensuring that the amount of decomposing organic matter is reduced and kept at a range that can be adequately managed through the cycling of nitrogen by nitrogenous bacteria. So, summarily, detritivores aid in achieving an efficient nitrogen cycle in the aquarium.

    What is a Detritivore Kit for the Nitrogen Cycle?

    A detritivore kit is basically a pack containing a variety of species that feed on decaying organic matter or fish waste found within the aquarium. In other words, the detritivore kit is a collection of detritus feeders that can be introduced into your fish tank. The kit is often prepared or stocked based on the capacity of the tank in which it would be used.

    To this end, the number of fish species – in relation to the waste that could be possibly generated – could also be factored in. That said, the detritivore kit can be packed with varying quantities of each of the (detritus-feeding) species it contains to adequately cater to your needs. It is also good to note that the species in the kit may differ in their detritus-feeding tendencies.

    Some of the common organisms you may find in the detritivore kit include:

    • Copepods: Copepods, in general, are quite versatile in that they can adapt to a wide range of feeding orientations. However, bottom-dwelling copepods like harpaticoids and zooplanktons are known to be very effective in clearing decomposing organic substances from the aquarium.
    • Nassa snail: Nassa snail will readily function as a detritivore in any aquarium, including nano tanks. It is particularly beneficial in cleaning sand substrate of decaying organic waste, and will not portend any danger to the tank’s inhabitants.
    • Brittle star: Brittle star belongs in the group of echinoderms, and is known to be an active detritivore, scrapping off dead remains from the nooks and crannies of the tank. It definitely makes a great addition in the kit, and the home aquarium as well.
    • Some other species that you may find in the mix of detritivores [as contained in the kit] are Gammarus shrimp, bristle worms, tigertail cucumber, bubble conch, and so on.


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