Beginners Guide to Cycling A New Fish Tank

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As fishkeepers, the nitrogen cycle is unavoidable. It’s the process every fish tank has undergone to establish a balanced aquatic ecosystem.

In this article you’ll learn how to successfully cycle your tank to ensure happy, healthy fish.

Let’s get started

What is the nitrogen cycle

So what exactly is the nitrogen cycle? You may have heard terms such as Nitrification Process, New Tank Syndrome, or Break-In Cycle being thrown around. All of these words point to one simple process going on inside every fish tank, the nitrogen cycle.

So what is it, and why is it important?

Imagine living in a room with all of your pee and poop, pretty disgusting right? Well, this is exactly how fish would live without the nitrogen cycle.

Whenever your aquatic friend’s decide to use the restroom, their waste is broken down into ammonia. And lets just say ammonia is the exact opposite of what your fish needs.

The nitrogen cycle is your fish tank’s way of filtering ammonia out of its system. Without this important ability, your fish would have really poor and dangerous living conditions.

So how exactly does the nitrogen cycle break ammonia down? Glad you asked!

Diagram of the nitrogen cycle

The Nitrogen Cycle builds beneficial bacteria inside your fish tank’s filter media that breaks down ammonia(toxic) into nitrite(toxic) into nitrate(not-toxic).

That’s the beauty of it right there! Beneficial bacteria growing inside your filter that takes care of your fish’s waste. Pretty cool if you ask me.

How long does the nitrogen cycle take

From my experience, a full cycle can take anywhere from 2 weeks to 6 months. The common rule is it’s done when it’s done.

There are just too many variables to make an exact prediction for how long the nitrogen cycle takes. Your tank size and population size are some metrics to look at.

Throughout the nitrogen cycle, you’re going to be measuring the amount of ammonia, nitrites, and nitrates in the water. The easiest way to do this is by purchasing an aquarium test kit. An aquarium test kit will allow you to test the amount of ammonia, nitrites, nitrates, and pH levels in the water.

During the nitrogen cycle, ammonia levels will rise and then drop as nitrite levels grow. You won’t see nitrate appear until your tank has a certain level of nitrite. Once nitrate levels start to surge, nitrite levels will drop.

When you can no longer detect nitrites, the nitrogen cycle has completed. It is now safe to add fish!

The three stages of the nitrogen cycle

Stage 1: The Rise of Ammonia

Through food or your fish’s waste, ammonia is introduced into the tank. There are two types of ammonia produced depending on your fish tanks pH levels. If your fish tank’s pH is below 7, ionized ammonia(NH4) is produced. Otherwise, you’ll get standard ammonia(NH3).

You’ll see a large surge in ammonia until bacteria starts to form. When you start seeing a decline in ammonia, nitrites form. You now enter the second stage.

Stage 2: The Rise of Nitrites

At this stage, bacteria has successfully grown inside your tanks filter. This bacteria breaks down the ammonia inside the water and turns it into nitrites. It’s common for your tank’s water to become cloudy during this stage.

Similar to the first stage, you’ll notice a buildup of nitrites until a colony of bacteria will form to dispose of them. You should notice nitrite levels rise at the end of the first week or during the second week. Once nitrite levels start to decline, you enter the third and final stage.

Stage 3: The Rise of Nitrates

This is the final stage of the nitrogen cycle. At this point, your tank should have a bacteria called nitrifying bacterium in your filter that breaks down ammonia into nitrite.

As nitrite levels drop, nitrate levels will rise. Nitrate is a fairly harmless chemical in low levels. A general rule of thumb is that if nitrate levels soar above 20ppm(parts per million) then your tank could become toxic.

An easy way to avoid nitrates polluting your fish’s water is to do regular water changes. I suggest you perform partial water changes(20-50% every 2 weeks, depending on your tanks stock levels).

Changing your fish tank’s water is also important as it removes substances such as DOCs (dissolved organic compounds), and replenishes minerals.

If you own a freshwater tank, you can take advantage of using plants to absorb some of the nitrates. On the other hand, saltwater owners can use live rocks and deep sand beds to absorb nitrates.

What if you don’t cycle your fish tank

The nitrogen cycle is a funny thing in that it’s going to happen whether you want it to or not.

Whenever you add any fish, they’re going to eventually leave waste which decomposes into ammonia.

In fact, this is the exact process of fish-in cycling – the method of cycling many choose that results in them putting their fish through the nitrogen cycle.

There’s one problem with the fish-in cycling method though, it’s dangerous! Ammonia is deadly to fish and is the primary reason many new fishkeepers report their fish dead a month after they get them.

So while your tank may cycle itself without any special effort on your end, there is no guarantee that your new fish will survive.

How to cycle your fish tank

You now understand what the nitrogen cycle is. So how do I start it? Before you actually start cycling your tank, you need to understand the two types of cycling methods.

The first type of cycling is fish-in cycling. This type of cycling utilises your fish’s waste to kickstart the ammonia growth. Only problem is that the ammonia and nitrite levels could rise to deadly levels before the nitrate comes into play.

Since you’re most likely a beginner hobbyist, I would steer clear of this way of cycling.

The second type of cycling is fish-out cycling. Unlike fish-in cycling, you’re not risking your fish’s life. To spike ammonia, you simply drop pellets of food, or add natural ammonia in the water. Fish food naturally decomposes into ammonia, thus starting the nitrogen cycle.

Which type of cycling should you choose? I always recommend going with the fish-out cycling method. This method requires little to no expertise to get into and produces a more stable tank in the long run. For some of you reading this article, you may have made the mistake of buying your tank and fish the same day. In that case, fish-in cycling is your only choice.

Don’t worry though, I’ll be covering both methods in detail!

How to cycle your tank without fish

You’re now ready to learn how to perform a fishless tank cycle. The first step is getting everything ready to start the process.

Equipment checklist

Before we start the fishless nitrogen cycle, we need to have the right equipment. When it comes to cycling without fish, you only need three products…

1. An aquarium test kit

The nitrogen cycle is an invisible process. With that in mind, you’ll need an aquarium test kit to see what’s happening inside your tank. I recommend going with a master test kit. These kits are useful because they allow you to test for many different chemicals.

It’s also useful to note that many local pet shops can do water testing. So check in with them to see if they will do it.

2. Ammonia or fish food

The first step of the nitrogen cycle is introducing ammonia. There are two main methods of doing this.

The first method is by adding flakes of fish food and letting them decompose naturally. The other, adding ammonia directly.

I’ve always added ammonia directly. This will make it easier to keep levels consistent, and speed up the process.

It’s important to use 100% pure ammonia. Most common household ammonia contain scents and additives, which will end the cycle before it even starts. Just stick to pure ammonia!

3. Dechlorinator

If you’re using tap water to fill your aquarium, be cautious of chlorine and chloramine. These two chemicals can kill the bacteria needed to cycle your tank.

Worry not! Using a good water conditioner when you do regular water changes can ensure that the bacterias will thrive. Remember, without the bacteria, the nitrogen cycle would not be possible.

When you have an aquarium test kit, a way of adding ammonia, and a water conditioner, you’re ready to cycle your tank!

Step 1: Setting up the fish tank

You know all the equipment you got with your tank? It’s time to set it all up.

Filter, heater, air pump, substrate, plants… get it inside the tank.

Why set up your fish tank?

The beneficial bacteria that we’re trying to grow needs a surface to stick to. Namely your substrate and filter media. In fact, almost all the bacteria will call your filter home.

Any electrical equipment, such as heaters, filters, bubblers, etc need to be turned on during the nitrogen cycle. You want to have everything running as if you were keeping fish inside. This will insure you have a faster cycle.

Step 2: Introducing ammonia

You now have your entire tank setup. Next step is to introduce ammonia to start the cycle.

You may recall that you can add ammonia into a tank by letting fish food decompose or by adding ammonia directly.

If you went with the fish food route, you want to add a few flakes into the tank every 12 hours. Although there are no fish, you want to add the same amount of food as if there were fish.

For those of you who went with the pure ammonia route, it’s time to add it. Read on the back of your bottle on how much parts per million(ppm) of ammonia is inside of 1 teaspoon. For example, at the time of writing this article, Fritz Ammonia produces 4ppm with one teaspoon in a 100 gallon tank.

For tanks less than 40 gallons: aim for 2ppm

For tanks larger than 40 gallons: aim for 4ppm

Once you add the ammonia to the tank, it’s time to test the levels of ammonia inside the water. You want to wait about an hour after adding the ammonia before actually starting the testing. Inside your testing kit, you want to use an ammonia test. If your tests come back with a lower ppm than your tank needs, add more ammonia. If your tests come back with a higher ppm, perform a water change.

Once you have the desired ammonia ppm levels, record the amount of ammonia you used.

This next part is going to test your patience, you’re going to be testing ammonia levels each day with the test kit. All you can do for now is wait until your ammonia levels drop.

Typically after a week ammonia levels will drop and it’s time to move to the next stage.

Step 3: Testing for Nitrites

At this point you’ve introduced ammonia into your tank by decomposing fish food, or by adding it purely. Once you see ammonia levels drop, it’s time to look at something new, nitrites.

You can test for nitrites with a nitrite test kit.

As soon as you start detecting nitrites inside your tank, you’ll know that your cycle has officially started. It’s important to continue adding ammonia throughout the cycle.

Step 4: Testing for Nitrates

After a couple weeks of testing for ammonia and nitrites, you’ll notice that nitrite levels start to drop. It’s time to test for nitrates!

When you start seeing nitrates, you have entered the final stages of the nitrogen cycle. Once ammonia and nitrite levels fall to zero, the nitrogen cycle is complete. However, if nitrate readings soar above 40, you’ll need to do some water changes to bring down this number.

Step 5: Adding your fish

When you can no longer detect ammonia or nitrite. It’s safe to add fish.

Girl pointing at fish in tank

Although you’re going to be excited that your tank is cycled, don’t add too much fish. It’s important to add a few fish at a time. I always recommend waiting a week or two before introducing more.

Consider cleaning any substrate before adding your first fish. Unwanted food may be lodged in there which could release deadly amounts of ammonia.

How to cycle your tank with fish

If you bought your tank and fish at the same time, you may not be able to go with a fishless cycle. Although more dangerous, a nitrogen cycle with fish can be completed safely.

Stage 1: Choosing the right fish

Some species of fish can handle a nitrogen cycle better than others. These special types of fish are called Hardy Fish. Your aim is to populate your tank with these fish to produce waste.

Recall that fish waste is broken down into ammonia. That’s how we are going to be introducing the ammonia into the tank.

You should have 1-2 fish per 10 gallons of water. Adding too much fish will lead to excess waste. This can cause deadly amounts of ammonia leading to your fish’s death.

Some of the best fish for cycling are:

  • Cherry or Tiger Barbs
  • Banded Gouramis
  • White Clouds
  • Most Minnows
  • Most Guppies
  • Zebra Danios
  • Pupfish
  • X-ray Tetras
  • Pseudotropheus

Stage 2: Feeding your fish

Feed your fish carefully and be sure not to overfeed them. From experience, feed your fish every two days. And make sure to provide them with small-medium size meals.


Simply put, when fish eat more they produce more waste. More waste means higher ammonia levels, which can be toxic for your tank.

Stage 3: Regular water changes

Regular water changes are a must. Changing the water in your tank regularly ensures toxins don’t pollute the water.

Aim to change 20% of the water every 2-3 days. Remove more and you risk removing ammonia the bacteria need to grow, thus ruining the nitrogen cycle.

It’s also important to use a water conditioner if you’re using the tap.

Stage 4: Testing for toxins

Like the fishless cycle, we’re going to be testing for ammonia, nitrites, and nitrates.

We want to initially test for ammonia until we see a steady drop, then test for nitrites. When nitrite levels drop, we then test for nitrates. When nitrate levels grow we want to test for all three chemicals. We know the nitrogen cycle is done when we cannot detect ammonia or nitrite levels anymore.

I generally test every day when I cycle new tanks, but you can get away testing every two or three days.

Stage 5: Adding more fish

Once ammonia and nitrite levels bottom out, you can start adding fish. Yay!

Mom and son in pet shop

But wait.

You’ll need to add new fish in gradually, introducing one or two fish at a time. On top of that, you’ll need to wait about a week before adding any new fish after adding fish.

The reason it’s important to add fish in slowly is because you risk spiking ammonia and nitrite levels. If this happens, your tank’s water becomes toxic.

Speeding up cycling times

For some, the nitrogen cycle can take up to 6 months to complete. So the question arises, can I speed up the nitrogen cycle? The answer is yes!

Using Live Plants

Live plants can introduce beneficial bacteria into your tank quickly. Especially if it’s from an established tank. An established tank is simply just a tank that has been cycled.

Just as plants absorb sunlight through photosynthesis, plants can absorb ammonia through protein synthesis. This makes ammonia less of a risk.

Using gravel from an established tank

If an established tank has an undergravel filter, you can actually use the gravel from the established tank in your new tank. This will kick start the growth of bacteria rapidly.

You want to take roughly a cup of gravel and hang it in a mesh bag inside your filter. This will allow bacteria to grow inside your filter, thus speeding up the nitrogen cycle.

Borrowing a filter media from an established tank

Most of the nitrogen cycle takes place inside your filter. Instead of waiting months for useful bacteria to grow inside your new filter, why not use the filter media from an established tank? This will allow the bacteria to multiply inside the new tank rapidly.

Seasoning your tanks filter

Take the filter that you’re going to use for your new tank and let it run inside an established tank for a week. What will happen is bacteria from the established tank will grow inside the filter.

After a week, migrate the filter to your new tank and boom, you just cheated the nitrogen cycle.

Beware of Cross-Contamination

There is a HUGE downside to using an established tank to speed up the nitrogen cycle. The downside is that you can catch some pretty nasty stuff that can be harmful to your new aquatic ecosystem.

It’s definitely possible that you could catch unwanted pathogens like harmful bacteria and parasites. Be sure to do your research on the established tank before you try any of these methods.

There’s no Perfect Solution

The reality is, there is no perfect solution to cycling your tank.

You can shorten the time. But, you still have to do regular water changes and regular maintenance if you want to be a successful aquatic parent.

Common Beginner Nitrogen Cycle Problems

If you are new to cycling, there’s a chance you may encounter some problems. Lucky for you, I’ve compiled some of the most common beginner cycling problems.

Extreme Algae Growth

Cycling your new tank is the perfect environment for algae to grow.

If you do notice an unusual growth in algae, be sure to limit the amount of light the tank is getting. If you’re using plants, I recommend you leave your fish tank lights on no more than 12 hours a day

Fish tank wont start cycling

For those of you who cycled with fish food or your fish’s waste, ammonia will rise around day 3. If by day 5 you don’t see a rise in ammonia, and you’ve ruled out the possibility of a faulty test, your tank isn’t cycling.

The problem is that there isn’t enough ammonia produced, or its being consumed too quickly. An easy fix to this problem is to add more fish food every 12 hours. Adding more fish food will create more ammonia.

Ammonia Poisoning

For all of you who opted in for the cycling method with fish(fish-in cycling) your tank could have ammonia poisoning. Some symptoms include:

  • Your fish lack movement, or seem lethargic
  • Your fish don’t eat, loss of appetite
  • Your fish tend to stay near the bottom of the tank
  • Your fish gasp for air at the surface

You can fix ammonia poisoning by doing regular water changes and feeding your fish less. Feeding less will leave less leftover food which will lower the ammonia levels.

Ammonia is not dropping(Fishless Cycling)

The main reason ammonia levels aren’t dropping is because your tanks pH level could be below 7. If your tank’s pH is too low, nitrifying bacteria(bacteria that makes nitrite from ammonia) cannot grow.

The easiest way to increase your tanks pH is to use a pH kit. Also, be sure to not use chlorinated water

Nitrate levels aren’t rising

The reason nitrate levels aren’t rising is similar to ammonia levels not dropping. Something is killing nitrate before it’s able to populate. Be sure that you aren’t using chlorinated water, or cleaning your tank vigorously.

The Nitrogen Cycle Never Stops

For all of us smart fishkeepers, the nitrogen cycle is unavoidable. If you want your aquatic ecosystem to thrive, you have to complete the nitrogen cycle first.

Even after your tank is fully cycled, the nitrogen cycle never stops. Bacteria will always break down the ammonia into nitrate.

Remember to always take water tests, as this helps you regulate a balanced ecosystem.

Photo of author
Hey there, my name is Gunnar Kennedy. I'm a fishkeeping enthusiast who's been in the hobby for over a decade now! I love sharing new ideas and helping others care for their aquatic friends!

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