What is the Nitrogen Cycle in aquariums?


What is the Nitrogen Cycle?

The fish and invertebrates that live in your aquarium produce waste on a daily basis. This waste includes poop, pee and uneaten food that is left in the tank.

This waste starts to decompose and releases ammonia into the water. Ammonia is very dangerous to your fish, and if it builds up in sufficiently large amounts, can even kill your fish!

Fortunately, the Ammonia is broken down by bacteria into Nitrites, which are a little less harmful to the fish. The Nitrites are then broken down by other bacteria into Nitrates, which are almost completely harmless. So basically, the Nitrogen Cycle is the process of growing enough of these ‘beneficial bacteria’ that the toxins in the water stay neutralised and at levels safe for your fish.

The Nitrogen Cycle process

Formation of Ammonia

As we have seen above, the inhabitants in the aquarium produce waste constantly. This is all organic matter that starts to decompose almost as soon as it touches the water in the tank.

The waste gets broken down into Ammonium and Ammonia. Ammonium is relatively less harmful to fish and is usually present when the pH is below 7. Ammonia, on the other hand, is very toxic to fish and is usually present when pH levels are above 7. We should try to keep the Ammonia levels in the tank at 0.

Formation of Nitrites

Ammonia is broken down into Nitrites by bacteria known as Nitrosomonas and Nitrospira. When you test the water and find that Ammonia levels are going down and Nitrite levels are going up, it means that the population of these bacteria is increasing. However, Nitrites are also toxic to the aquarium inhabitants and need to be eliminated.

Formation of Nitrates

The good news is that you don’t need to do much to eliminate Nitrites. As the level of Nitrites increases, another species of bacteria shows up, called Nitrobacter. Nitrobacter break Nitrites down in Nitrates, which are a lot more tolerable to your fish and invertebrates. Nitrates are not a problem unless their levels are very high. As long as you are performing water changes regularly, the Nitrate levels will be in check.

When the Nitrite levels begin to fall and the Nitrate levels begin to go up, you know that the Nitrobacter bacteria are growing in number.

You want all 3 species of ‘beneficial bacteria’, also called ‘nitrifying bacteria’ (Nitrosomonas, Nitrospira and Nitrobacter) to be present in your aquarium in large numbers. These bacteria will not only reduce the toxins in the water, but they can also control ammonia spikes and other sudden changes in water quality. This will greatly reduce the stress on your fish.

What cycling your aquarium means

 When you cycle your tank, you are increasing the population of these 3 bacteria, so that they can process the toxins and your fish and invertebrates have a safe environment to stay in. Now you know why it is necessary to cycle the aquarium. It is a lot more important to cycle the aquarium before adding any fish or inverts for the first time. If there are no nitrifying bacteria or their numbers aren’t sufficient, the ammonia level will rise very quickly and kill your fish and inverts.

Ways to cycle your Aquarium

You can cycle your aquarium without fish or with fish. Let’s look at both methods in detail.

Cycling your aquarium without fish

It is recommended that you cycle your tank before you get any fish or other creatures to put in the tank. To do so, you will need an aquarium water test kit, dechlorinator, and Ammonia or fish food.

First of all, you need to set the aquarium up. Place the substrate, heater, filter, artificial decorations, air pump, etc. inside the tank. You need all of these in place because the bacteria you want to grow live on the surfaces of things in the tank. The more surface they have available, the more they are likely to grow. Then, add water to the aquarium.

Once you add water to the tank, use the water testing kit to check the water parameters. Find out what the water hardness level, pH level, ammonia and nitrite levels are. If you’re using tap water, add the dechlorinator in proportion to the amount of water in the tank. Then add the Ammonia or some fish food. If you are using Ammonia, only use 100% pure Ammonia. Other Ammonia variants contain additives that will hinder the cycling process.

Make sure the water level is higher than the heater and filter (if the filter is submersible). Then turn on all electrical equipment, i.e. filter, heater and air pump. Keep the heater’s temperature in a range that your future fish will thrive in. If you have a particular species in mind, click here to read about their water parameter requirements, including the temperature range they need. Keeping the water heated will also make the bacteria grow faster.

The bacteria will start showing up in about a week and decompose the fish food or process the Ammonia, depending on what you choose to put in the tank. You will need to keep adding more fish food or Ammonia every 1 or 2 days, or the bacteria will exhaust the food supply and begin dying out. Remember, our goal is to increase the bacteria population.

Keep testing your pH and Ammonia levels at least once every 2 days during the cycling process. The pH level shouldn’t go below 7, otherwise the cycling process could stop. You can click here to read how to increase the water’s pH level. Once the Ammonia levels start going down, also start testing the Nitrite levels, which should be increasing.

When the Nitrite levels start increasing, the Nitrobacter bacteria will appear. After that, the Nitrite levels will begin to decrease and the Nitrate levels will begin to increase. You can check the nitrate levels using the water testing kit. If Nitrates are present, it means the Nitrogen Cycle is almost over.

Add more fish food or Ammonia. If the Ammonia and Nitrite levels are zero 24 hours later, your Nitrogen Cycle is complete. Congratulations! Now you can add your fish!

If you will add the fish 1 day later (or more), you will need to keep adding the fish food / Ammonia, to keep the bacteria fed. Once the fish are added, you don’t need to take special care of the bacteria.

Cycling your aquarium with fish

This is not the preferred method to cycle the aquarium, as it is dangerous for your fish. The fish will get stressed out, and that can lead to infections and diseases. The fish may even die. I’m only writing this for those who may have bought the aquarium and the fish together (like I did when I bought my first fish), so they can increase the chances of their fish’s survival.

If you haven’t bought your fish yet, cycle your tank first using the fishless method mentioned above. And only get the fish once the cycle has been completed.

Just like the above method, our aim is to first establish, and then increase the beneficial bacteria population in the aquarium. You should keep the water temperature in the range that is comfortable for your species of fish. Click here to read about the suitable water parameters for your fish. Like the above method, the pH level should not go below 7. Here are ways to increase the pH levels, if they are decreasing. Needless to say, the water testing kit is indispensable for this method as well.

If you just wait for the bacteria to appear on their own, it could take about a week. It is too long, and all the fish could die by then. You will need to add the bacteria to the aquarium. Thankfully, this is not very difficult. If you know people who have aquariums for at least 1 month or so, you can ask them for some substrate from their tanks for a couple of weeks.

The best thing you can do is take the filter media from you friend’s tank, and put it in your tank. When you remove the filter media from their tank, stir the substrate up nicely (don’t do this if the substrate is sandy). Doing so will throw up more bacteria into the water, which will attach to other surfaces. This will reduce the impact of removing an entire bacterial colony from the tank.

Take a bucket or something with you so when you are travelling from your fiend’s place to your home, you can keep the filter media submerged under water. This water should be the same water from the aquarium it was in.

Place the filter media in your tank and squeeze it multiple times, so all the dirt and everything comes out and into your tank’s water. Then move it around in your tank so it gets rinsed. When you do this, a lot of the bacteria will have come inside your tank. Now they just need to attach themselves to your tank’s substrate or filter media, where they will multiply. You’ve just reduced the cycling time by a week! When returning the filter media to your friend, be sure to keep it wet throughout the journey, as there will still be some bacteria left in it. You don’t want them to dry out and die. When they are returned to their tank, they will also repopulate.

The reason for this is that beneficial bacteria grow on surfaces of substrate and filter media. A friend’s established tank will already have beneficial bacteria growing in it. Adding the substrate or filter media will immediately add an entire colony of all 3 species of these bacteria. So as soon as the organic matter is broken down into Ammonia, it will also get broken down into Nitrites and then Nitrates.

Along with these steps, you need to take some other steps as well, to cycle the tank faster. Your fish will need to be fed sparingly during the cycling process. Feed them just once a day. The quantity should bas much as they can eat in about 3 minutes. When the fish are fed so little, they will poop little, so the bacteria can neutralise the toxins. There won’t be any excess food either, so it won’t dirty the water.

You need to keep testing the water for pH levels, Ammonia, Nitrite and Nitrate levels every single day. If the Ammonia or Nitrite levels become high, you will need to do a water change of about 20%. Before adding new water to the tank, always dechlorinate it by either adding a dechlorinator, or letting it sit in a bucket for 24 hours. If Chlorine is not removed, it will kill the beneficial bacteria, ruining the entire cycling process.

When the Ammonia and Nitrite levels stay at zero 24 hours after you’ve fed your fish, you have sufficient bacteria in the aquarium to neutralise the toxins. Congratulations! The fish are safe now.

If you want to add more fish, you will need to add them one or two at a time. This will give the bacteria sufficient time to increase their population. If too many fish are added to the tank, the Ammonia levels will spike, which will either cause great stress to the fish or kill them, defeating the purpose of the cycling the tank slowly in the first place.

Also be wary not to overcrowd the aquarium. Doing that will add a tremendous amount of bioload to the aquarium. You can click here to read more on what bioload is and how to increase your tank’s capacity to handle more bioload.

What if I don’t cycle my aquarium?

Well, if you don’t cycle the aquarium, it is still going to cycle anyway. Organic matter will still be in the aquarium, and beneficial bacteria will still form. The problem is, if you leave the tank to cycle on its own, the organic matter will release too much Ammonia to be processed quickly enough. You won’t know what the water parameters are, and one day you will wake up to find your fish dead. Then you will wonder why they died ‘all of a sudden’, as they were doing fine until last night. This is the most common mistake beginner fish keepers make. So the cycling process needs to be done, even if it takes up to a month to complete.

Syed Baseeruddin Hyder

I’ve been keeping fish and invertebrates in aquariums for over 5 years. Over the years, I’ve kept more than 15 different species of fish and invertebrates. Through ParadiseInATank.com, I hope to guide new and experienced fish keepers alike with as detailed information as I can get.

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