What does tank cycle mean? This term may sound familiar to you, but in this article you will learn exactly what it means. What is the nitrogen cycle? Why is it so important? And how do I know my tank is cycled? This is a simple guide on how to cycle your aquarium.
When we talk about tank cycling process we are primarily referring to the nitrogen cycle, biological cycle or nitrification process.
The nitrogen cycle is a natural process that helps break down toxic metabolic products into less harmful substances for fish. You can’t avoid the nitrogen cycle when owning your own fish tank.
When you setting up an aquarium, you need to remember that nitrogen cycle normally lasts between 6 to 8 weeks. The aquarium water should reach a biological balance before you add a new fish.
Every young aquarist should be aware of how the nitrogen cycle proceeds. Lack of knowledge may lead to mass or single fish deaths, algae growth, or cause cloudy water in fish tank. Incorrect water filtration or too many fish in the tank may lead to biological imbalance.
Key Takeways:
- The nitrogen cycle is crucial for a healthy fish tank, breaking down harmful substances.
- It typically takes 6 to 8 weeks to establish a biological balance before adding fish.
- Fish waste releases ammonia, converted by beneficial bacteria.
- The nitrogen cycle consists of Ammonia, Nitrites, and Nitrates stages.
- Regular water changes maintain safe nitrate levels.
- Two cycling methods: Cycling with Fish (experts) and Cycling without Fish (beginners).
- Speed up with mature tank substrate, filter media, or plants.
- Regular testing ensures a safe tank.
- Watch for ammonia poisoning if levels are too high.
- Blackwater tanks mimic Amazon ecosystems.
- Ich (White Spot Disease) can affect freshwater fish.
- 3-gallon and 1-gallon tanks are unsuitable for fish.
What is biological balance?
Biological balance is a condition in a tank that maintains a balance between the biocenosis and the biotope in a way that ensures the welfare of the animals and plants kept in the tank. For simplicity, we can say that it is a situation in which the aquarium is able to cope with excessive fish feces and organic matter in the tank.
I show you what is going on behind the scenes.
No fish wants to swim in water surrounded by own pee and poo. It can harm their health. We must remember that fish poop and pee in the same water they swim in. We poop in the toilet, fish don’t have that option.
Poop and other waste releases harmful ammonia into the water. Beneficial bacteria help them to destroy ammonia. The beneficial bacteria that are needed for the cycling process live in the filter media, substrate, and decorations.
Ammonia is toxic to your fish. Death by poo? No way! The cycling process helps avoid above situation.
The nitrogen cycle process
The nitrogen cycle is easiest to explain in the diagram.
The source of nitrogen, which is an essential element in the tank, is basically ammonia. But nitrogen cannot be used by itself. First of all ammonia (toxic) must be converted into nitrites (toxic), and then into nitrates (non-toxic).
Your fish eat the food you prepare for them. When you feed your fish, a lot of food sinks to the bottom (carbon dioxide is also produced during this process – see the diagram).

Cycling process starts with waste. Uneaten fish food and rotting plants all give off ammonia as they break down. The ammonia will build up inside your tank, reaching toxic levels.
Nitrogen cycle is a three-stage process in which fish are kept alive and healthy by their friendly bacteria.
A working nitrogen cycle is one that is in balance. During the process of converting ammonia into nitrites, and then nitrates, harmful bacteria are able to build up. This process is called NITRIFICATION.
Nitrogen is essential for growth and life. If plants don’t get enough nitrogen they cannot grow and the cycle of nature will end if you fail to keep up.
Nitrification is the process of converting ammonia into nitrite, followed by the process of oxidizing the nitrite to nitrate in two stages. It involves separate organisms or direct ammonia oxidation to nitrate by commamox bacteria.
The nitrification process consists of three stages: Ammonia, Nitrite and Nitrate.
STAGE 1: Ammonia (toxic)
In the first stage, there is an increase in ammonia. Ammonia and ammonium ions are primarily formed by the breakdown of proteins from excess food, feces, dead plant parts and dead fish. In the first stage of the cycle the ammonia content reaches toxic levels.
When your ammonia levels begin to drop you know that you’ve got these bacteria in your tank, usually after a week (process can take 7 to 10 days).
At pH values above 7, toxic ammonia is formed in the water. Less harmful ammonium are present at pH values below 7.
The level of ammonia in a freshwater tank from 0.2 to 0.5 mg/l (ppm) is already harmful to fry and young fish. Levels above 0.5 mg/l are fatal to fry and very harmful to adult fish.
The amount of ammonia will keep increasing until the bacteria in your fish tank start to form.
When your system is functioning properly, it’s able to use all available nutrients in the waste and recycling your tank.
However, if your system isn’t operating as it should, the ammonia will continue to build up or remain constant.
You know you’ve entered the second stage of cycling process when ammonia levels start to decline.
STAGE 2: NITRITES
Bacteria called Nitromonas oxidize ammonium ions and ammonia to nitrite. Oxygen is also required for this process (because Nitromonas bacteria are an aerobic organism).
This process can be written using the following equation:
NH3 + O2 → NO2− + 3H+ + 2e−
As a result of this process, nitrites are formed. The decrease in ammonia content is related to the increase in nitrite concentration. When ammonia levels begin to drop, nitrite levels begin to rise. Both of these chemicals are toxic to your fish.
Even a small level of nitrites can lead to fish poisoning, cloudy water or disturbing the biological balance in the tank. This is an important moment in the nitrogen cycle.
You should know something about new tank syndrome (nitrite peak).
The new ‘tank syndrome’ describes rapidly rising nitrite levels in water. In higher concentrations, nitrite is harmful to fish and can be fatal. An aquarist should take action at a nitrite concentration of 0.5 mg/l. In fact, nitrite is even more deadly than ammonia. Generally, in a tank with a good biological balance, the nitrite content should be 0. If the nitrite content declines, you know that good bacteria have developed in the tank.
In the next stage of nitrogen cycle, nitrites are converted to nitrates.
STAGE 3: NITRATES
Nitrite are oxidized by bacteria called Nitrobacter, which can be represented by the following equation:
NO2− + H2O → NO3− + 2H+ + 2e−
When the nitrite levels decline, you will see that the nitrate levels rise.
Nitrates are the main source of nitrogen for plants. In addition, plants absorb CO2 and release the oxygen needed for life by all organisms in fish tank.
The final product of the nitrogen cycle are nitrates.
For most plants, a nitrate level of 5 to 10 mg/L is sufficient.
In mature tanks there is usually no need for supplementation. This is because nitrates are formed by the break down of protein substances, and are the end product of the nitrogen cycle. Concentrations above 80 mg/l cause inhibition of plant growth and rapid algae growth.
The accumulation of large amounts of nitrate can even become toxic to fish. We can quickly lower the nitrite value by changing the water.
Regular water changes (once a week) are in fact intended to prevent excessive nitrate levels in fish tank.
How to cycle your aquarium?
We can cycle aquarium in two ways:
Cycling with fish – this is a method recommended for experts only as it can poison your fish; if you add fish to the tank right away, the breakdown of food and poop will initiate this cycle causing an increase in ammonia. The increase in ammonia and nitrite is dangerous to your fish. If you do not take the correct steps the water environment will become toxic and the fish will begin to die. Those that survive however will be more susceptible to disease.
Cycling without fish – also called fishless cycling method. This is the most reliable process, but it is also time-consuming and requires patience. Animals are added to the tank some time after setup, preferably in stages. A few small fish produce low ammonia concentrations, allowing enough time for the nitrogen cycle to settle into equilibrium.
Cycling without fish (fishless cycle)
Stage 1: Start the ammonia growth
Bacteria need a surface to cling to. And your substrate and filter media is where they live. Make sure that you keep your equipment on during the cycling process.
You need to check the temperature of your water, make sure it is at a comfortable level. The beneficial bacteria in your tank will grow faster if the water is between 65-85 degrees Fahrenheit.
Once the tank has been flooded you need to think about how to initiate the ammonia growth. After all, there are no fish in the tank yet. A good way is to add few fish food to the aquarium. We can do this several times every 12 hours. The food flakes will start to decompose releasing the ammonia into thetank.

You should check the pH of your tank water before proceeding.
Your tank is acidic because of the bacteria that produce acids. If you don’t correct the problem, you’ll eventually have a low pH. All that’s needed to raise the pH and get the cycling process going is a simple 20% water change.
Stage 2: Ammonia test
We absolutely need to buy ammonia tests. It’s pretty darn important to monitor the levels of ammonia. Test your aquarium water every couple of days to monitor the levels of ammonia.
The ammonia level should be greater than 3 ppm. At the very beginning, the ammonia concentration will increase and this is perfectly normal.
If the ammonia concentration is too low, add more fish food and let it decompose. If your ammonia levels are higher, it means that you need to change your tank water. Perform a water change by replacing 20% of the tank water with new, ammonia-free water.
Let’s test the water every other day. It is worth keeping ammonia at 3 ppm – then we are sure that the Nitromonas bacteria start using ammonia. Let’s test ammonia for about a week.
You will need to look at how this information fits together, and it will help you through the rest of the steps.
Stage 3: Test for nitrite
We already know that as ammonia increases, Nitromonas bacteria begin to grow. These bacteria break down ammonia to nitrite. At this stage, we need to get a nitrite test. We can use a commercial test kit for this.
By detecting nitrite in the tank we are sure that the cycling process has started.
We are going to feed the bacteria a little more food. Make sure your ammonia levels remain under 5 ppm.
Keep in mind that nitrite levels will rise in the initial stages. Once it reaches a peak it will begin to drop.
Stage 4: Test for nitrates
After some time (a few weeks) you will notice that your nitrite levels will begin to drop. As we remember this is related to the formation of bacteria that break down nitrites to nitrates. At this stage we should get a nitrate test. Test the aquarium water regularly.
If we detect nitrates we can be sure that the cycle is coming to an end. When the ammonia and nitrite levels are 0 it means that the cycling process is complete. If the nitrate level is above 40 it is worth making a few water changes to bring this number down.
Step 5: Adding fish
Now that your tank has been cycled, your bacteria colonies are large enough to remove the ammonia and nitrite that are produced by the fish waste. This nasty stuff isn’t going to harm your fish any time soon!
If our tests no longer detect ammonia or nitrite, and the nitrate concentration is at the correct level, we can begin adding fish.
Before adding the fish you should remove any decaying food from the medium. If you leave them out, ammonia may appear again in your tank. You want to perform a water change to remove any excess ammonia and nitrite. That means you will need to replace the water in your tank with clean, fresh water.
Remember not to add many fish at once. It is advisable to spread the settling-in process over time. In the first stage it is advisable to let in a few fish, and in a week or two more.
Cycling with fish
While it’s possible to remove the ammonia and nitrites from the water, this is not the preferred method of raising them. You’ll expose your fish to ammonia and nitrites during the process and a lot of fish won’t be able to make it through.
This isn’t going to be the best option for you. But If you’ve accidentally bought your tank and fish on the same day, this may be your only option.
Stage 1: Introducing the right fish
In the first stage, we let a small number of fish into the tank. The purpose of adding the fish into the tank is to produce ammonia, which causes beneficial bacteria to grow.
Important!
You should allow fish into the tank that can survive the high ammonia and nitrite levels long enough. Otherwise, your fish may simply die. There should be 1-2 fish per 10 gallons. However, remember that the fewer fish you let in, the better.
Here are some fish that can survive a high concentration of ammonia:
- Minnows
- Guppies
- Cherry Barbs
- Tiger Barbs
- Banded Gourmis
- White Clouds
- Zebra Danios
- Pupfish
- X-ray Tetras
- Pseudotrophius Zebra
Stage 2: Feeding fish
Feeding the fish is a delicate process. If you overfeed the fish, they will produce a lot of waste. This increases the level of toxins in your tank before the good bacteria is able to colonize your tank.
If you have a lot of food at the bottom of the tank, it can lead to a buildup of ammonia.
It’s important to feed your fish the right amount of food at the right times. Too much food can cause waste to build up, which can lead to ammonia and other toxins.
You should feed your fish twice a week, providing them with moderate-sized meals.
Stage 3: Water changes
Regular water changes are a must. Fish will die if they become toxic, so removing toxic substances will help ensure the fish remain healthy. You’ll want to regularly test your water for ammonia and nitrite, and then regularly change it. The goal is to have 10-25% of the tank water replaced every 2-3 days.
Any more chlorine or chloramine and you’ll kill the good bacteria and ruin the process. You should use RO water or de-chlorinator. Chlorine or chloramines may kill the beneficial bacteria in your tank.
It is also worth using filter media that absorb ammonia. You should only use special media until the cycle stabilizes. If there is no ammonia in the water, the bacteria will have nothing to eat.
Stage 4: Test kit
If you want to keep your fish healthy, you’ll need to purchase a test kit so you can monitor the levels of ammonia and nitrite.
Nitrite is a common toxin found in fish tanks. Keeping an eye on your ammonia and nitrite levels can help you keep your fish healthy throughout the process of setting up your tank.
The ammonia monitoring equipment in the tank is important because you will want to know when the ammonia levels rise back above the safe operating range. While it’s true that you can test every day, you don’t really have to do so. You can easily make it work with testing every two or three days. When the levels drop back down to 0 this means that the cycling process is complete.
The ammonia level should be 0, the nitrite level should be 0, and the nitrate level should be no higher than 40 ppm.
Testing is important because you’ll want to know when your cycling process is over.
Ammonia poisoning
Ammonia is harmful to your fish. If its level is too high ammonia poisoning can occur.
Symptoms of ammonia poisoning:
– Lack of movement
– Loss of appetite
– Pathological behavior (lying at the bottom of the tank, catching air at the surface)
– Red streaks in the fins
– Inflamed of the eyes, gills or anus
In case of observing alarming symptoms in the first place you have to change water to lower the ammonia levels. Lack of any reaction may lead to the death fish.
Algae bloom
Algae bloom can be a problem in any new tank.
If you have a problem with algae blooms, try turning off the aquarium light for a few days. You can’t just turn off the light if you have plants in the tank!!!
You should avoid over-fertilizing your tank, use a CO2 injector, and keep your lights at a minimum of 10 hours a day.
New tank not cycling, why?
If you don’t see an increase in ammonia after 4-5 days you need to consider what could be the cause. The lack of ammonia indicates that your tank is not cycling.
Let’s check you have the correct test kit. It may be out of date and not detecting ammonia levels.
If the tests still do not detect ammonia perhaps you should add more ammonia to the aquarium. It may also be related to too many plants. In this case you can remove some of the plants. If we are not able to do this we should add more ammonia.
How do I know my tank is cycled?
After testing your aquarium water for ammonia and nitrite and nitrate. If the results come back with 0 ammonia, 0 nitrite and some nitrates, your fish tank is cycled. Concentrations of nitrate should not be above 20 – 30 ppm.
Ammonia not dropping (fishless cycling method)
The ammonia concentration does not go down mainly for 3 reasons:
– using chlorinated water – chlorine kills all the beneficial bacteria in your aquarium. If there are no bacteria they will not be able to break down the ammonia and it will stay in the fish tank all the time. It is a good idea to add a sanitizer to the water or use RO water.
– Intensive cleaning – if you clean your aquarium too often (decorations, stones, gravel) you remove beneficial bacteria. This applies in particular to the filter, where bacteria also multiply. You should not clean it while cycling.
– Too low pH – if your water has a pH below 7 ammonia is converted to ammonium. Nitrifying bacteria do not feed on ammonium. In this situation we should increase the pH in our aquarium.
Maybe ammonia is reducing but just very slowly?
Why are nitrate levels not rising?
This is of course because the ammonia level is not dropping. If the ammonia level is not dropping it means that the nitrifying bacteria have not started to multiply or are dying before they can act.
How long does it take to cycle your aquarium?
A new fish tank will need 6 to 8 weeks of cycling before it is ready for fish. Depending on what method you use for growing your bacteria, the exact length of time will vary.
The first step in introducing a new fish into your tank is to make sure the ammonia and nitrite levels are within acceptable limits before introducing more fish.
How can I cycle my fish tank faster?
In order to make the nitrogen cycle run more efficiently we can use various methods. You can use your existing tank to to accelerate the cycling process. Just seed the tank.
Use substrate from a mature tank
If you are lucky enough to have an established aquarium, its substrate will already contain an excellent amount of bacteria. The substrate in your tank provides a surface for beneficial bacteria and acts as part of your aquarium’s filtration system.
Since this is so important for the health of the fish, it is wise to use substrate from mature tank because is a source of good bacteria.
The first way is to hang a mesh bag full of the old substrate to the filter system. You can also place a substrate from the existing tank across the gravel in the new tank.
You can also use a few plants from a mature tank. Bacteria on their leaves can help to accelerate the cycling process in a new setup.
Using filter media from a mature tank
If you have access to an existing tank, you can use filter media from it that is already in the filter. The filter media in the established aquarium already have nitrifying bacteria so you won’t have to wait for natural colony reproduction. However, you must remember to use filter media from a tank of a similar size and a similar number of fish.
Connecting a new filter to a mature aquarium
It’s a good idea to fix filter using in new tank next to the filter in an mature tank.
This process will cause the beneficial bacteria colonize in the new filter system. These two systems should run alongside each other for about a week or two.
Add plants from an existing tank
Live plants have bacterial colonies on their leaves. Adding few plants to the tank will definitely speed up the cycling process of the new setup. Use fast growing plants such as Vallisneria and Hygrophila. These plants will help to absorb more ammonia what is called protein synthesis. Protein synthesis can reduce ammonia levels in aquariums, making them safe and stable for fish.
IMPORTANT!!!
One of the biggest problems with using seeding material is that the water parameters of the two tanks have to be very similar for the process to work. The first rule is to make sure the pH and KH levels of your two tanks are similar. The second rule is to remember that new fish always bring a little bit of their own bacteria with them.
Adding nitrifying bacteria colonies to the water or to the substrate can also speeding up the process. Such colonies are available in the form of ready preparations that should be added to the water or to filter system.
While some over-the-counter supplements can help to increase your fish tank’s levels of beneficial bacteria, they’re not a complete solution. Your product also needs to be checked that still has good viability by checking that it’s within the expiration date.
Remember that using an existing tank to accelerate the nitrogen cycle in aquarium may be associated with certain risks. An existing aquarium may contain pathogens, harmful bacteria and parasites. If we are not sure whether the existing aquarium is healthy, do not perform any treatments. This can lead to displacement and contamination with harmful organisms.
In conclusion
It doesn’t really matter what method you use. There is no magic way that will make the cycling process go very quickly. You can shorten the time by testing the water, water changes and regular conversation.
Nitrogen cycle in aquarium never ends. You need to understand that the nitrogen cycle is an ongoing process. Although you can’t see it, it is designed to protect our fish from dangerous substances. The maturation process of the tank is an element that we cannot skip.