The Aquarium Nitrogen Cycle – Beginners Guide & Diagram

Understanding the aquarium nitrogen cycle is the most fundamental part of the fish-keeping hobby. You have to learn this if you want to successfully keep fish. But, you don't have to be a chemistry nerd to understand how it all works. It's actually pretty straightforward...
Aquarium Nitrogen Cycle
Aquarium Nitrogen Cycle

Don’t be one of those people who quits this wonderful hobby out of frustration because their fish died within a month.

Understand the aquarium nitrogen cycle process, cycle your tank properly and avoid running into new tank syndrome.

This guide will cut out all the complicated stuff and explain it in simple terms that everyone can understand.

What Is The Nitrogen Cycle For Aquariums?

The nitrogen cycle, also known as the nitrification cycle, is a process where naturally occurring nitrifying bacteria (beneficial bacteria) convert toxic substances in your aquarium water into a much less harmful substance.

It basically converts toxic water into safe water.

How and why this occurs is because fish are just like us, what goes in one end must come out the other end!!

When we’ve done ‘our thing’, we flush, and away it goes. Fish don’t have that option, they have to swim around in their own toilet.

Their poop then starts to break down in the water and releases ammonia, which is highly toxic to fish.

Ammonia will burn their skin and gills, which can lead to fin rot, and eventually death.

On top of that, any food they missed during feeding, dead tankmates, or any dead plant matter trapped in the tank will start to rot and release ammonia.

The nitrogen cycle is nature’s way of dealing with it all.

However, in a new aquarium, there aren’t enough beneficial bacteria to get rid of these toxins quickly enough.
It takes time to establish bacteria and stabilize the cycle.

Stages Of The Aquarium Nitrogen Cycle

These are 3 stages of the aquarium nitrogen cycle.

1 – Ammonia – NH₃

Fish poop, leftover food, and rotting vegetation begin to break down and release ammonia into the tank water.

Ammonia is extremely toxic to fish.

2 – Nitrite – NO⁻₂

Once you have ammonia in your water, naturally occurring nitrifying bacteria called Nitrosomonas will begin multiplying on your aquarium’s surfaces.

These bacteria consume ammonia and convert it into nitrite.

Although nitrite is less toxic than ammonia, it’s still deadly to fish.

3 – Nitrate – NO⁻₃

Now you have nitrite in your water, there is another form of beneficial bacteria called Nitrobacter, that will consume nitrite and turn it into nitrate.

Nitrate is far less harmful to fish, but it still needs to be kept at safe levels.

You keep nitrates at safe levels by cleaning your aquarium and performing partial water changes when needed.

Nitrogen Cycle Aquarium Diagram

It may be easier to visualize with this fish tank cycling chart:

Aquarium Nitrogen Cycle Diagram
The nitrogen cycle process in an aquarium

How Long Does The Nitrogen Cycle Take In An Aquarium?

The nitrogen cycle in an aquarium typically takes around 2-8 weeks to complete. It all depends on which method of cycling your tank you choose to use.

Water temperature and pH levels also play a role in how long it takes.

How To Speed Up The Nitrogen Cycle

I have a guide on how to cycle a fish tank fast here, see method number 1.

How Do You Maintain The Nitrogen Cycle In An Aquarium?

You can maintain the nitrogen cycle in an aquarium by:

  • Not overstocking your tank
  • Not overfeeding your fish
  • Removing dead fish and invertebrates
  • Removing any rotting plant matter
  • Testing your water regularly
  • Cleaning your aquarium and performing partial water changes when needed.
  • Increasing your biological filtration, if needed

How Do I Increase My Biological Filtration?

Fluval BioMax Ceramic Rings, Amazon Sword, Marineland Bio-Filter Balls
Fluval BioMax Ceramic Rings, Amazon Sword, Marineland Bio-Filter Balls

As long as you have cycled your aquarium correctly and the beneficial bacteria in your tank can convert the ammonia into nitrate as quickly as your fish can produce it, you shouldn’t need to increase the biological filtration.

However, if you do need to increase the biological filtration one option is to add some biological filtration media to the filter, such as ceramic rings or bio-balls.

These products increase the surface area for beneficial bacteria to grow on which will reduce the amount of ammonia, nitrite, and nitrate in your aquarium.

If you’re having difficulty keeping the nitrate levels under control, another option is to add some plants to your aquarium, not only do they look great, but they also feed off the nitrates in the water.

Fast-growing plants such as water sprite, and tall-growing plants like Amazon Sword, will work well.

Pothos is another great option, even though it’s actually a houseplant. It does an amazing job at removing ammonia and nitrate from your tank water. I have an in-depth guide on using pothos in aquariums.

Once the plants have gobbled up the nitrates in the water, you will need to feed them with a product such as API Leaf Zone to keep them healthy.

API Leaf Zone

Nitrogen Cycle In An Aquarium Summary

Here’s the aquarium nitrogen cycle process summarized in 5 steps:

  1. Fish poops in the water.
  2. Fish poop and other organic matter release ammonia.
  3. Beneficial bacteria grow and convert ammonia to nitrite.
  4. A different kind of beneficial bacteria converts nitrite to nitrate.
  5. You remove nitrates with water changes and plants.

Hopefully, you now have a better understanding of the nitrogen cycle and you can go and successfully cycle your fish tank and get it ready for your new fish to thrive in.

Find A Fish Vet Near Me

If you would like to seek veterinary advice you can find a specialist aquatic vet in your area using the links below:

Check out the Aquarium Health section for more guides just like this one.

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Hey, fellow fish enthusiast 👋! I'm Paul and I've been an aquarium addict since I won my first goldfish at the fair many years ago. Let me share with you the knowledge that I've gained along the way (and avoid the mistakes I've made!), so you too can create an underwater paradise for your aquatic friends.