How to cycle a (freshwater) fish tank

Beginner, intermediate or advanced, it doesn’t matter what fish keeping category you’re in, knowing how to cycle a tank is important. Introducing the nitrogen cycle to your aquatic environment is one thing that cannot be overlooked, a process that can often be daunting for first-timers. But fear not, step by step, this article will teach you how to cycle your freshwater tank confidently. Making sure you have a healthy environment for your fish to thrive in.

The nitrogen cycle, the what and the why? 

I’m sure some will remember the term from those Biology lessons as a child, but what exactly does it mean? To put it in layman’s terms, the fish tank is going to be your fish’s world. They will eat, rest (fun fact fish don’t sleep), and of course, need to poop in the tank. As I’m sure you’ve guessed, toilet paper still hasn’t burst onto the aquatic scene. Therefore the fish’s excrement is going to be floating around the tank, which breaks down, releasing dangerous ammonia into the water that over time can kill your fish. 

Thankfully, this is where our friend, the nitrogen cycle, comes in to save the day. It’s a natural three-stage process, where you get beneficial bacteria to grow within the tank and filter, protecting your fish from the dangerous ammonia. 

What is beneficial bacteria?

A healthy aquatic environment will rely on beneficial bacteria to break down the harmful waste in your aquarium turning this waste into less toxic compounds. Today many filters come with bio rings or bio-media, which provide surfaces for the beneficial bacteria to live on. The bacteria love oxygenated water movement, which is why the filter area is their favored destination. 

Ok, I’m listening, what’s the process?

Stage 1: Ammonia 

In essence, ammonia is the product of all the different types of waste within a tank that has been broken down, such as fish excrement, rotting plants, or uneaten food. Once fully formed, beneficial bacteria will eat the ammonia. However, until that happens, the ammonia will continue to build and in some cases rise to a dangerous level. You will know when beneficial bacteria are present in your tank when the ammonia levels begin to decline; usually, this will happen after a week or so. 

Stage 2: Nitrites 

The nitrite levels will begin to rise as you see the ammonia levels decline. This works as the beneficial bacteria eating the ammonia gives of this new chemical, nitrite. However, at this stage, we are not entirely in the clear, since this chemical is still very harmful to your fish. As the nitrite levels continue to rise in a similar way to stage one, new bacteria will be born, Nitrobacter. When large enough in their numbers, they will begin to eat the nitrites and omit the chemical, nitrate. 

Stage 3: Nitrates 

Nitrate forms the third and final stage of the nitrogen cycle. When the nitrite levels start to drop this will be a clear indication that nitrates are present in your tank, which are far less harmful than nitrite and ammonia. However, in large quantities, they still can become toxic. This is why you do a water change to help mitigate this risk by lowering them back to a harmless level. (Nitrate levels up to 40ppm is considered a safe environment for your fish to live in) 

Nitrogen cycle (needyfish)

How long does it take to cycle a tank? 

There is no right answer here, and to be completely honest, it will take as long as need be. It’s essential to give your tank enough time. Generally speaking, you are probably looking at anything between six to eight weeks to have a tank that is fully cycled. 

What happens if I don’t cycle the tank?

Ultimately, the nitrogen cycle process will happen whether you intentionally do it or not. The excess food and excrement from your fish will break down to ammonia, which will, in turn, start the cycle. However, the question that should be asked is, how will your fish fair if you skip this process and release them into your tank. 

As we explained earlier, ammonia levels will continue to build up in the tank until enough beneficial bacteria has formed to eat the ammonia. However, you are going to have an extremely toxic environment that will be very detrimental to your fish’s health until you have a tank that is omitting nitrate, which, therefore, completes the cycle. 

Imagine being in a swimming pool with friends, immersed in each other’s excrement, and adding to the matter of not being able to leave. Sounds pretty unpleasant, doesn’t it? Well, this could be the fate of your fish if the tank is not cycled correctly. 

Do I cycle the tank with or without fish?

For those of you who have already brought the fish, no need to panic! Since it’s entirely up to you whether you decide on a fishless cycling method or not, both are possible. However, you are certainly going to have more work when cycling the tank with fish. Ultimately, their lives are at stake, and if not done correctly (I don’t mean to be dramatic), this could result in their unhappiness and death. So my recommendation would always be to cycle the tank without fish, removing any added pressure. I will, however, explain how to do both in more detail below. 

What do I need to cycle a tank without fish?

1. An aquarium test kit

Any standard’ aquarium test kit’ will do. The nitrogen cycle is invisible, so you will need a test kit to understand what stage you’re at. The kit will be able to test the water for ammonia, nitrite, and nitrate. 

2. Dechlorinator

Water that comes out of a tap will have been treated with either chloramine or chlorine. These are chemicals that can damage your beneficial bacteria and, in turn, affect your fish. It is essential to use a dechlorinator whenever you add tap water to a tank. 

3. Ammonia

Adding fish food and letting it decompose works. But alternatively, you can use pure ammonia when cycling your tank. This saves you the time waiting for the waste to breakdown and also allows you to keep the ammonia levels constant.

How to cycle a (freshwater) fish tank - (needyfish)

Step by step: cycling a fish tank without fish

Step 1. Set up your tank

Make sure your tank is completely set up. All those extra bells and whistles you brought, such as the filter, heater, and substrate, should be up and running. Why? Well, this is because beneficial bacteria need a surface to grow on. The constant running of the filter creates the oxygenated water movement, that’s the perfect condition for the beneficial bacteria to thrive. 

Step 2. Check PH levels of tank water

Making sure the PH level is at the correct level before starting the cycle should not be overlooked. For context, the PH scale runs between 0-14. Most freshwater/ tropical aquarium fish will fair best at a PH of 6.8-7.6. However, some other fish may require lower or higher levels. If the levels drop below 6, this can affect the whole nitrogen cycle since the beneficial bacteria will start to falter; therefore, not processing the ammonia in the water. If this happens, a 20% water change will usually be enough to raise the PH levels again.

Step 3. Add the Ammonia

Make sure to read the instructions of the pure ammonia you plan to use. As it will specify how many drops per gallon should be used for each size tank. Check out this gallon calculator if unsure how much water your tank holds. 

Once added, let it sit for an hour giving it time to make its way around the tank. You will then want to use your testing kit to see if the right amount of ammonia has been added. The kit will give the measurements in ppm. If it is lower than the recommended amount, merely add more, and if too high, you will need to perform a water change, which will help lower the ppm. 

It may differ depending on what brand of pure ammonia you decide to use. But those with fish tanks that are less than 40 gallons should aim for 2ppm, and those more than 40 gallons aim for 4ppm. 

Once you’ve got your tank to the correct ppm-level, this is all that can be done for now. Continue to test once a day, and when you see the levels starting to drop, you will know the cycle has begun to take effect. Give or take, this phase should take around a week. 

Step 4. Check for Nitrites

Once a week has passed, it will be time to check for nitrite, be patient. If detected, the cycle has officially started. Since you don’t have any fish in the tank, remember you are the only one at this point supplying the ammonia. It may differ depending on the ammonia you are using, so have a look at the instructions. I would advise adding half of what you added on day one to continue to feed the nitrites. Remember to keep the ammonia levels below 5ppm. Like the previous step, you will need to monitor the nitrite levels daily through continuous testing. Initially, you will start to see them rise, and when they begin to drop, you can move onto the next phase. 

Step 5. Check for Nitrates

Remember to continue to add ammonia daily (half your initial day one dose) to keep the levels above one ppm. Using your testing kit, now that you have seen a drop in nitrite. You will need to confirm that this is because of the introduction of nitrate. If detected, you are now in the final stage of the cycle.  Continue to test daily. The nitrogen cycle will be complete when you can add ammonia, and within 24 hours, when you test the nitrite and ammonia levels again, they have returned to zero. 

Step 6. Time to add your fish!

Well done for getting to this stage and putting all that hard work in. I know it takes time, but you will have now successfully made a safe environment for your fish to thrive in. Try and resist the urge to put lots of fish in all at once; consider easing them in gradually alongside continuous testing. 

How to cycle a (freshwater) fish tank - (needyfish)

What do I need to cycle a tank with fish?  

1. An aquarium test kit

You will not be able to see the nitrogen cycle visually, so you will need a test kit to understand what stage of the cycle you’re at. Most kits will be able to test the water for PH levels, ammonia, nitrite, and nitrate.  

2. Water conditioner

As I previously mentioned, water conditioners can be used for treating chlorine and chloramine. They can vary, so check before purchase, but some can also neutralize ammonia, nitrite, and nitrate in water for 24-48 hours. If you’re planning to do a fish-in cycle, you will be getting very familiar with this product. ‘Seachem prime’ is a popular choice of water conditioner. However, there are many brands out there that also do a great job.

Step by step: cycling a fish tank with fish

To be clear, we would not recommend cycling a tank this way, due to the high toxic levels your fish will be exposed to. It’s not very humane; however, if you have already bought the fish, this is the method to follow!  

Step 1. Set up your tank

Like we explained earlier, you need to make sure your tank is all set up. The heater, filter, substrate, all need to be operational. This is because the beneficial bacteria will need surfaces to grow on, most importantly, the substrate and filter.

Step 2. Introduce your fish

The more fish you put in the tank will speed up the process. Wrong! The more fish you introduce will mean, the faster the build-up of ammonia. Ultimately an overload of fish at this stage will lead to excess waste. I would recommend per 10 gallons of water, adding 1-2 fish. Hardy fish, or more commonly known as starter fish, are considered the fish of choice for this role. 

Step 3. Don’t forget to feed your fish

I would recommend feeding your fish every two days. Naturally, the more you feed them, the more waste they are going to produce, which will ultimately turn into ammonia. Be mindful of this since you won’t be doing yourself any favors by overfeeding your fish. 

Step 4. How to control the ammonia 

Once you have tested ammonia in the tank, it is time to use the water conditioner I mentioned earlier. Check the instructions as it will specify how much will be needed and how frequently for your size tank. It essentially binds the ammonia, nitrite, and nitrate for 24-48 hours, making them entirely harmless for your fish. However, after that time has passed, they will be able to harm your fish again. Hence the need for a top-up. If you have any doubts, you are always better to add too much than too little conditioner.

During this period, it’s important to continue testing your tank for the levels of ammonia. Once you’re getting a 2ppm reading for ammonia, I’d recommend doing a partial water change, which will further lower the ammonia levels. It is essential to repeat this process until you see the slow down of the rise in ammonia. At which point, you will be ready to move onto the next step. 

Step 5: Testing for Nitrites, oh! and don’t forget conditioner 

At this stage, you will want to start testing for nitrite. You will now have two harmful chemicals in the form of ammonia and nitrite to contend with.  So continuing to use the water conditioner is vital! To work out the correct dosage, merely add both the readings for ammonia and nitrite together, and if the combined readings reach a total of 4ppm, like the previous step, you will need to do a partial water change. Eventually, if you keep repeating this process, you will notice the ammonia readings drop to zero. It’s important to continue testing, and once the nitrite levels stop rising as quickly, you can move to the next step. 

Step 6: Testing for Nitrates and a little more conditioner! 

Chances are if the nitrite levels have stopped rising as quickly; you’re going to test positive for nitrate. However, if you don’t, merely continue what you were doing in the previous step. When nitrate becomes present, the finish line is in sight! Continue to test and use the water conditioner; the ammonia levels should be at zero with the nitrite levels declining. When both the nitrite and ammonia levels are at zero, congratulations, you have completed the cycle! 


I hope this article has now given you a clear idea of how to cycle your tank with and without fish properly. The cycle will now be self-sufficient to continue to run in the background. However, we would still recommend running routine tests on the ammonia and nitrate and nitrate levels, making sure nothing goes wrong as slight spikes are inevitable. It is considered safe for fish to live in a nitrate environment of 40 ppm or less. We would, however, recommend doing partial water changes to prevent exceeding this number. As always, if you have any questions, please do not hesitate to contact us. 


My aquarium won’t start cycling!

If you have been using the pure ammonia liquid I recommended, the chances of this happening are rare. However, the common reason for the cycle not starting is when either something is eating the ammonia too fast before the bacteria have time to get to it or there is no source of ammonia. Try either adding more ammonia or removing some plants from the tank and wait 12 hours to see if this makes any difference.

What are the signs of ammonia poisoning and what to do? 

How to cycle a (freshwater) fish tank - (needyfish)

This is not good news for your fish and will put their lives in danger. Common symptoms include: 

  1. Bleeding gills
  2. Red streaking on the body and fins
  3. Fish laying at the bottom of the tank 
  4. Fish appears darker in color 
  5. Fish gasping for air at the surface of the water

Immediately reducing the ammonia level is the only way you can treat this. If the damage is not too extensive, the fish can recover. To reduce the levels of ammonia, we would recommend doing a series of small water changes.

Why is my ammonia not dropping?

The reason this will be happening will be because your PH levels are too low. A 20% water change can help remedy this. 

Help Algae Bloom!

This is an issue that can often happen. To address this turn your tank lights off, or if your tank is by a window since natural light can often encourage growth, consider moving the tank. Check out our article here on the common causes of algae and how to prevent it. 

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