How to Cycle an Aquarium (Explaining The Nitrogen Cycle)

by Kevin

Nitrogen cycle explained, fish tank cycle

What is the Nitrogen Cycle?

Nitrogen cycle graphic

The nitrogen cycle is a natural waste management system performed by certain species of bacteria.

In an aquarium, the nitrogen cycle is responsible for creating a safe and healthy environment for your fish.

A more sciencey definition for the nitrogen cycle is the biochemical process in which nitrogen is converted into different chemical forms as it circulates through the air, soil, and water.

Scientists often refer to the nitrogen cycle in its terrestrial context, where bacteria convert atmospheric nitrogen into biologically usable forms that are absorbed and used by plants (this process is known as “nitrogen fixation”).

The nitrogen cycle is vital to agriculture and the production of crops because plants need help from nitrogen fixing bacteria in the soil to get access to usable nitrogen.

The cycle works as follows:

1) Nitrogen fixing bacteria convert atmospheric nitrogen into ammonia.

2) Nitrifying bacteria then convert that ammonia into nitrites, which are converted into nitrates.

3) Then denitrifying bacteria convert nitrates into atmospheric nitrogen.

And so the circle continues.

The Aquarium Nitrogen Cycle

Nitrogen cycle in planted aquarium.

In an aquatic environment, like an aquarium, the same process occurs.

But, we as fish keepers care most about step 2, in which ammonia is converted into the less toxic nitrites and nitrates.

While plants are able to process ammonia, it is fatal to fish in large quantities.

So, as fish keepers, we care most about creating suitable conditions for nitrifying bacteria to colonize and grow.

The confines of an aquarium permit ammonia to build up quickly or spike, especially as fish waste and uneaten food accumulate in the tank.

Nitrifying bacteria are vital for keeping ammonia levels under control, promoting fish health and allowing aquarists to perform less frequent water changes.

If you are setting up an aquarium for the first time, it’s important to understand how long it takes a tank to cycle because adding fish or invertebrates before a tank is fully cycled can lead to disaster.

Nitrogen Cycle Stages in Home Aquariums

Alright, let’s say you waited two weeks (or more), and you put fish into your tank.

The nitrogen cycle is still important after you introduce fish into the tank because fish produce ammonia.

With fish, the nitrogen cycle operates as follows:

Stage 1

pH chart in color

Aquarium fish produce waste (feces) and uneaten food builds up at the bottom of the tank.

The uneaten food and fish waste breaks down into either ionized ammonium (NH4) or un-ionized ammonia (NH3), depending on the pH level of your water.

Ammonium results if your water’s pH is under 7 (acidic) and ammonia is produced if your water’s pH is higher than 7 (alkaline).

Remember, 7 pH is neutral. And numbers lowers than 7 (ex. 6, 5, 4, 3, 2, 1, and 0) are acids, and numbers higher than 7 (8, 9, 10, 11, 12, 13, 14) are bases or alkalines.

Now, ammonium is not particularly harmful to aquarium fish, but ammonia is toxic.

Stage 2

Bacteria (called nitrosomonas) become established in the tank and begin oxidizing the ammonia in the tank, essentially making it harmless.

But, the oxidization of ammonia produces a different problematic substance, nitrites.

Nitrites are also toxic to fish and need to be eliminated to establish a healthy environment for your fish.

If you are testing your water, you may notice that nitrite levels rise at the end of week 1 or week 2.

Stage 3

At stage 3, bacteria called nitrobacter establish themselves and begin converting nitrites into nitrates.

Nitrates are a far less toxic substance for fish, but should be managed by performing consistent water changes.

At this stage, your tank is established and cycled, but your tank still needs you to remove the high levels of nitrates that will continue to build up over time.

Anaerobic bacteria—bacteria that grow in the absence of oxygen— are able to break nitrates down into harmless nitrogen gas, but in fish tanks these bacteria are generally nonexistent.

Aquarium plants will absorb and remove some of the nitrates in a freshwater tank, and live rock will remove nitrates in a saltwater tank, but consistent water changes are the primary way for the aquarist to remove nitrates.

Nitrogen Cycle Timeline

While nitrifying bacteria will begin multiplying as soon as you setup your tank, the colony will not be large enough to process a significant amount of ammonia for about 2 to 6 weeks, depending on tank size and surrounding conditions.

But, there are methods for helping your tank cycle properly.

How to Cycle A Fish Tank

The most basic way to cycle a tank is to fill it up with water and wait about 2-4 weeks for the necessary bacteria colonies to establish themselves.

But, there are better ways to cycle a tank that will help the beneficial bacteria establish themselves more quickly and allow you to add fish sooner.

1) Add an Aquarium Filter

Many people may not realize this, but aquarium filters are meant to incorporate bacteria as the biological component of the filtration system.

Bacteria establish themselves in the filter as a colony.

Every time water passes over these bacteria, they convert harmful substances like ammonia and nitrites into less harmful substances.

Filters also provide a dark, safe place for bacteria to colonize where they can’t be disturbed.

Adding filter media to a filter provides additional surface area for bacteria to colonize and will help your tank cycle faster.

Even if you only plan to keep a betta fish, having a filter for your betta will make maintaining the aquarium easier.

2) “Starter” Fish Method

Using starter fish to begin the nitrogen cycle is not ideal and may actually kill the fish.

The fish often become stressed by the high levels of ammonia and nitrites, and may succumb to disease from weakened immune systems.

Certain fish are better at handling these conditions than others but there are major drawbacks to this cycling method besides killing fish and losing money.

I remember (back in the day) my sister and I asking our parents every other day about when we would be able to add fish to our new 110 gallon.

It had only been cycling for about a week.

My dad tried to persuade us to wait but we somehow persuaded him that we should ask the pet store if there were any fish we could add “now.”

To our delight the pet store clerk steered us into buying zebra danios and black skirt tetras.

While the fish survived the tank cycling ordeal, I remember the mindset that we were left with: these fish were just “starter” fish, we would get the “real” fish when the tank was fully cycled.

Later we did purchase other fish, and those hardy danios and black skirts looked out of place, and we would tell our friends that those were the leftover “starter fish.”

That’s not a good mindset to have toward beautiful living creatures.

So, beware that cycling a tank with fish may not be a good start for those new to the aquarium hobby.

And better methods exist for cycling a tank.

3) Cycling a Tank Without Fish

The basic idea of fishless tank cycling is to add ammonia to the tank to jumpstart the nitrogen cycle.

Many commercial products exist that give specific dosing instructions, but DIY nitrogen cycle methods are also effective.

a) Adding Pure Ammonia

Pure ammonia can be added to a fish tank at a dosage of about 5 drops per 10 gallons of water.

But commercial products like Ammonium Chloride for aquarium treatment may be a better option for the beginning aquarist.

Commercial products often come with detailed instructions for how to properly cycle your tank without fish.

Generally, the aquarist will need to dose ammonia daily until testing reveals significant nitrites.

Then, dosing can be reduced until testing detects nitrates.

When nitrates are present, perform a 30-40% water change, and your tank will be ready for fish.

b) Adding Live Nitrifying Bacteria

Dosing live nitrifying bacteria from the start is an excellent way to cycle a tank quickly.

Live nitrifying bacteria can be purchased from sites like Drs. Foster and Smith or your local fish store may carry it.

Many aquarists have had success with Tetra Safe Start, Bio-Spira, and Dr. Tim’s.

c) Fish Food

Adding flakes of fish food to your tank is a cheap DIY method for cycling a tank.

As the fish food decomposes, it releases ammonia and jumpstarts the nitrogen cycle.

However, adding fish food may create a new headache for the aquarist.

Aquarium fish food contains numerous nutrients, including phosphate, which encourage algae growth.

Your tank may cycle properly, but you will be left battling all kinds of algae before you even add fish.

If you do want to cycle your fish tank with fish food, buy bunches of live aquarium plants and float them in the tank.

These plants will compete with the algae for nutrients and prevent it from flourishing.

d) Chunk of Raw Shrimp or Fish

Not the most glamorous method for cycling an aquarium but the decaying meat will release ammonia as it decomposes.

Personally, I’m not a fan of this method because I do not like seeing a chunk of raw meat in my aquarium.

e) Add Gravel and/or Filter Media From a Cycled Tank

If you already have a cycled tank, or have a friend or neighbor who does, adding substrate or filter media from the established tank is a quick way to introduce a colony of nitrifying bacteria to your new tank.

But, be sure to ask if medications were dosed in that tank. Copper is found in a number of fish medications and is harmful to shrimp and invertebrates.

If you are unsure, you can test the water for copper before deciding to use that gravel in your new tank.

Another great way to acquire live nitrifying bacteria is to find someone with a sponge filter and have them squeeze it out into a container for you.

They can continue using the sponge filter and you get a dose of bacteria – win, win.

Speeding Up the Nitrogen Cycle

While it may be tempting to add fish before a tank is fully cycled, doing so will likely cause “New Tank Syndrome.”

New tank syndrome is simply a fish’s reaction to elevated levels of toxic substances like ammonia and nitrites.

If you do not want to wait a full 6 weeks for your tank to cycle, consider methods for speeding up the nitrogen cycle, which include adding nitrifying bacteria via commercial products and using gravel and filter media from established tanks.

A few other factors that can help you cycle your tank quicker are:

1) Add Water From an Existing Tank

If you have access to water from an established aquarium, simply fill your new tank with that water and test it after about a week see if nitrates are present.

This method is a great way to get a tank up and running in a couple weeks.

2) Take a Filter from an Established Tank

Much like using gravel or filter media from an old tank, taking a filter from an established tank and placing it into your new tank will drastically increase the speed at which the new tank cycles.

3) Increase the Temperature of the Tank

Generally, a temperature between 80 degrees F and 82 degrees F will encourage the beneficial bacteria to grow faster.

4) Add Java Moss or Moss Balls

Java moss and moss balls (actually algae balls) from established tanks will contain all kinds of bacteria and other beneficial organisms that will speed up your tank’s nitrogen cycle.

Both do well in low light conditions and provide grazing areas for shrimp and fish.

And they look great too!

Maintaining a Cycled Aquarium

Once your tank is cycled with healthy colonies of bacteria removing ammonia and nitrites, you are ready to add fish.

Many fish keepers (including myself) are tempted to add “all” the fish at this point.

But that is unwise.

Add Fish Slowly

Instead, add two to three fish to the tank every week.

Gradually adding fish allows the bacterial colonies to adjust to the influx of ammonia produced by the new fish.

Add too many fish at once and the bacteria may not be able to handle the bio-load, leading to fish sickness or death.

Don’t Kill the Bacteria

1) When performing water changes, add de-chlorinator to the new water before pouring it into your tank.

2) If your filter is sluggish or becomes clogged, do not rinse the filter media with regular tap water.

Chlorine in tap water will kill the nitrifying bacteria and bring you back to square one of the nitrogen cycle: waiting for bacteria to grow.

3) Also, cleaning tank décor with water from that tank rather than water from the tap will prevent beneficial bacteria casualties.

Use Live Plants

Live plants not only provide a place for bacteria to colonize, but plants also absorb nitrates from the water column.

So, in heavily planted aquariums, live plants can reduce the need for water changes and will act as a buffer against large swings in water chemistry.

How to Monitor the Nitrogen Cycle

To observe the nitrogen cycle for yourself, buy an aquarium test kit that tests for ammonia, nitrites, nitrates, and pH (the hardness or softness of your water).

Nitrogen Cycle Steps Summary

Ok that was a lot of info. Here is a quick summary of the steps you need to take to properly cycle your tank:

1) Fill your tank with de-chlorinated tap water.

2) Pick a method for adding ammonia to the tank (remember fishless cycling is much kinder and controllable).

3) Test your tank’s water periodically for ammonia, nitrites, and nitrates.

4) Once nitrates appear and nitrites vanish, do a 20% water change.

5) Add fish! (2-3 at a time).

I hope this explanation helps you feel confident about cycling your own tank.

If you are trying to decide which fish to keep in your tank after it’s cycled, check out my article on best fish for a 10 gallon tank.

Stay zen aquarists.