5 Signs of a Cycled Aquarium: A Guide To the Nitrogen Cycle

What is a cycled aquarium? In simple terms, it means your fish tank has completed the nitrogen cycle.

The nitrogen cycle makes the water safe for your betta by reducing harmful toxins.

Fish waste, uneaten fish food, and decaying organic matter increase ammonia levels.

Beneficial bacteria convert this harmful ammonia and nitrite into less harmful nitrate.

Most aquarium owners do a fishless cycle. But a fish-in cycle is also possible.

So, what are the signs of a cycled aquarium?

When the nitrogen cycle is complete, water parameter tests show no signs of ammonia or nitrite and very little nitrate. The water is clear, and there are no signs of excessive algae growth. A cycled tank keeps your betta healthy and active and creates a balanced bioload.

The Nitrogen Cycle and Beneficial Bacteria

What Is the Nitrogen Cycle?

Organic matter in the tank releases ammonia in the tank as it decays.

In the absence of a nitrogen cycle, ammonia can reach dangerous levels.

An established nitrogen cycle prevents ammonia from reaching toxic levels in the aquarium.

The nitrogen cycle does not begin until the formation of nitrogen bacteria in a new tank.

Beneficial bacteria then get to work by converting harmful toxins into non-toxic substances.

This cycling process continues, keeping the water clean and safe for your betta.

It takes anywhere from 2-6 weeks to four months for an aquarium to cycle.

The timeline for a completed cycle depends on how soon the beneficial bacteria colonize and the amount of toxins in the water.

Beneficial Bacteria and How They Work

Beneficial bacteria are tiny organisms living on surfaces within a fish tank.

The beneficial bacteria begin by oxidizing ammonia into nitrite. Ammonia and nitrite are very toxic for bettas and other aquatic life.

Once the bacteria convert the ammonia into nitrite, they begin the next stage of the process.

This stage involves oxidizing nitrite into nitrate. Nitrate is still toxic for bettas, but tolerable in low amounts.

Live plants also feed on nitrate during the process of photosynthesis. The plants then release oxygen into the water during the day.

Some forms of beneficial bacteria convert ammonia into nitrate in one step instead of two.

Establishing a Colony of Beneficial Bacteria

Beneficial bacteria are present in every fish tank.

These bacteria thrive on oxygen, carbon dioxide, and ammonia.

Live plants and ammonia supplements supply these nutrients when there are no fish in the tank. Oxygen is as important as ammonia in the reproduction of nitrifying bacteria.

Beneficial bacteria live on flat surfaces, such as:

• Substrate
• Plant leaves
• Biological filter media

The bacteria do not multiply in open water, only on flat surfaces.

Beneficial bacteria have a slow reproduction process. It can take several weeks or months before a colony of bacteria forms.

You may speed up the colonization process in two different ways.

Commercial products containing beneficial bacteria are available in fish stores and online shops. These beneficial bacteria bottles contain a variety of freeze-dried bacteria.

Add the correct dose according to the directions on the label for an instant boost in bacteria.

Another method is using filter media from an already established tank.

Up to 80 percent of a tank’s beneficial bacteria live in the filter media. This number is even higher in well-cycled tanks with crystal-clear water.

Beneficial bacteria need a higher flow rate than most aquariums provide. Since the filter media has the highest flow rate in the tank, it makes an ideal home for the bacteria.

This method is only safe if the established tank has optimal water parameters. Otherwise, you increase the risk of adding harmful pathogens to your tank.

Using either of these methods can speed up the establishment of bacterial colonies by several weeks.

Types of Beneficial Bacteria

Several species of bacteria feed on nitrogen compounds.

These bacteria fall into one of two categories:

• Aerobic bacteria
• Anaerobic bacteria (Heterotrophic sp.)

Aerobic Bacteria

Aerobic bacteria need oxygen for survival.

There are two groups of this type of bacteria. They include ammonia-oxidizing bacteria and nitrite-oxidizing bacteria.

Some examples of ammonia-oxidizing bacteria are:

• Nitrosomonas spp.
• Nitrosococcus spp.
• Nitrosospira spp.
• Nitrosolobus spp.
• Nitrosovibrio spp.

These bacteria convert ammonia into nitrite.

Examples of nitrite-oxidizing bacteria include:

• Nitrobacter spp.
• Nitrospira spp.
• Nitrococcus spp.
• Nitrospina spp.

As you might have guessed, these bacteria convert nitrite into nitrate.

Aerobic bacteria oxidize ammonia and nitrite for energy. They use carbon dioxide for building their structure with carbohydrates and proteins.

This process is how they thrive and multiply.

Anaerobic Bacteria

Anaerobic bacteria are much different than aerobic bacteria. But they still play a crucial role in the nitrogen cycle.

The Heterotrophic species of anaerobic bacteria can create food from organic materials.

These bacteria convert nitrate into nitrogen gas without using oxygen.

Anaerobic bacteria live in lower levels of the substrate, where there is no oxygen.

Water Testing and Parameters

Why Water Testing Is Important

Testing your aquarium water lets you know the toxin levels at every stage of the nitrogen cycle.

If your water tests show high levels of ammonia and low nitrite levels, your tank is in the first stage of the nitrogen cycle.

During the second stage, ammonia levels decrease and nitrite levels increase.

In the final stages of the aquarium cycle, nitrate levels appear, while ammonia and nitrite are nonexistent.

Your tank’s pH levels are also important.

If your pH levels are lower than 7.0, the nitrogen cycle can stall.

You must continue testing your water parameters every week. This ensures you have a healthy environment for your betta and other aquatic animals.

Parameters To Test For in a Betta Tank

The most important parameters you must test for in a betta tank include:

• pH
• Ammonia
• Nitrite
• Nitrate

If you have issues with your pH levels, test your general hardness (GH) and carbonate hardness (KH). The GH and KH of your water affect the stability of your pH levels.

Another important parameter is the water temperature.

Betta fish need stable water temperatures at all times. Sudden temperature changes can cause shock.

Ideal Ranges for Each Parameter in a Betta Tank

Ideal water parameters for betta fish are the following:

Bettas prefer neutral to slightly acid pH levels. Any variations outside of the ideal parameters can harm your fish.

Acidic water causes burns and erratic behavior in your betta.

In alkaline water, your betta is more prone to illness. Alkaline water also encourages algae growth.

Ammonia and nitrite poison betta fish and cause severe health problems.

The presence of nitrates in the water is safe at low levels. Aquatic plants keep nitrate levels low by feeding on them.

Bettas come from a tropical climate and need warm water temperatures for survival.

Low temperatures cause digestive issues, which can lead to other health conditions.

Hot water decreases oxygenation in the tank and your betta can suffocate.

Types of Water Testing Kits

The two main types of water testing kits for aquariums are test strips and test tube kits.

Test strips are easy for beginners. You dip the test strip in your aquarium water and compare it to a color chart.

While test strips are inexpensive, they have some downsides.

These types of kits are usually limited in what they measure. They are also less accurate and sometimes difficult to read.

Test tube kits measure the widest range of parameters with more accurate results. But, they are more expensive and not always easy to use.

You fill a test tube with tank water and add drops of the appropriate solution. Then, compare the color of the water in the test tube with the included color chart.

I recommend the Nutrafin Master Test Kit as the best test tube kit for aquariums.

This kit measures 10 different water parameters, including GH and KH. The kit has high accuracy and includes a case for all the testing components.

Water Changes and Maintenance

Importance of Regular Water Changes

Fish waste is the biggest source of ammonia in an aquarium. Decaying leftover food and dead plants also contribute to increased concentrations of ammonia.

Beneficial bacteria work hard at lowering the amount of ammonia in the water. But without regular water changes, your tank can suffer from ammonia spikes.

Weekly water changes keep nitrate levels within safe parameters, as well.

Water changes also replenish vital minerals and maintain stable pH levels.

Frequency and Amount of Water Changes

Maintain a healthy environment for your betta by performing a weekly partial water change.

Remove only 20-25% percent of the water at one time.

Taking too much water out of your betta tank can remove beneficial bacteria. This disrupts the nitrogen cycle.

You may need several water changes if you have an ammonia spike with your fish in the tank. Performing a partial water change for several days in a row makes the water safe for your fish again.

Do not do large water changes all at once. This only makes the problem worse by disrupting your nitrogen cycle.

In extreme cases of ammonia toxicity, remove your betta to a quarantine tank until your water stabilizes.

Prolonged exposure to ammonia can cause severe health issues in your betta.

Your betta may need treatment with antibiotics or antibacterial medications.

Tips for Safe Water Changes

If you use tap water in your aquarium, you must add a water conditioner. Water conditioners remove chlorine and chloramine, which are toxic to bettas.

Do not use distilled or reverse osmosis water. These types of water do not contain essential minerals your betta needs for good health.

Ensure the new water is at the same temperature as the water in your tank.

Sudden changes in water temperature can cause shock in your betta. Bettas usually recover from temperature shock, but sometimes it kills them.

Before removing any water from the tank, turn off your aquarium filter and heater. This removes the risk of equipment malfunction and electrical shock.

Other Maintenance Tasks

During a water change, use the gravel vacuum to clean the substrate. This removes fish waste, uneaten food, and other debris.

Use gentle movements, so you do not disturb the substrate too much. Disturbances to the substrate can cause cloudy water conditions.

Also, use this time to remove algae on the sides of the tank. Magnetic algae scrapers make this task easy.

Accumulations of algae steal essential nutrients from the water column. This is not healthy for your betta or live plants.

If you have algae eaters in the tank, do not remove the algae unless there is more than they can eat.

Remove dead plant leaves so they do not release harmful bacteria as they decay.

Check your filter media for excess debris. This debris can clog the filter and reduce its efficiency.

Never rinse filter sponges or other filter media in tap water. This removes the beneficial bacteria from your filter media.

Protect your beneficial bacteria colony by rinsing your filter sponge or cartridge in tank water instead.

Signs of a Cycled Aquarium

Clear and Odorless Water

Ammonia and decaying organic matter cause foul odors in your tank water.

During the early stages of the nitrogen cycle, bacteria particles give your tank water a cloudy appearance.

The water is clear and free of foul odors in a cycled aquarium.

If your betta tank has an earthy odor, do not worry. This is common and means you have a healthy environment in your aquarium.

Stable Water Parameters

Water parameters fluctuate a lot during the nitrogen cycle.

It starts with high levels of ammonia. As ammonia declines, nitrite levels increase.

In the final stages of the nitrogen cycle, there are high levels of nitrate.

Once the aquarium cycles, water parameters become more stable.

Water tests should show zero levels of ammonia and nitrite with low levels of nitrate.

Aquatic plants feed on the nitrates. Weekly water changes also keep nitrate at acceptable levels.

Absence of Algae Growth

Algae blooms are common in the final stages of the nitrogen cycle.

These algae blooms signal high levels of nitrate in your aquarium water.

When nitrate levels become more stable, algae growth decreases.

Aquarium plants and regular water changes help lower nitrate levels even further.

Depending on your aquarium lighting and the number of plants, you may still see algae in small amounts. This usually happens because the light is too bright for the number of plants in the tank.

Decrease the occurrence of algae by adjusting your lights to the appropriate level for your plant density.

Healthy and Active Fish

Your betta’s appearance and behavior say a lot about its environment.

In a cycled tank, your betta stays active and healthy. Signs of a healthy betta include bright coloring and a healthy appetite.

If your betta is lethargic, not eating well, or has red streaks on its body seek help immediately. These are all signs of ammonia poisoning.

Treatment for ammonia poisoning involves administering medications to your betta. You must do this in a separate quarantine tank.

Antibiotics and antibacterial medications prevent open wounds from becoming infected. Without prompt treatment, ammonia poisoning can kill your betta.

Balanced Bioload

A balanced bioload means your tank is not overrun with more waste than it can handle.

In a cycled tank, fish waste from a single betta is insufficient for sudden water parameter changes.

The beneficial bacteria break down the ammonia and nitrite before it has a large impact on your aquarium water.

A small tank with many fish has difficulty completing the nitrogen cycle because of the heavy bioload.

Fishless vs. Fish-In Cycling

Fishless Cycling

Fishless cycling is the process of completing the nitrogen cycle without any fish in the tank.

This method takes longer but avoids exposing your fish to dangerous toxins.

Since no fish waste is added to the tank every day, you must add an ammonia supplement.

The ammonia supplement jumpstarts the beneficial bacteria into action.

Fish-In Cycling

Fish-in-cycling involves completing the nitrogen cycle with your fish in the tank.

This is sometimes referred to as the “guinea pig fish method.”

The fish creates waste, which feeds ammonia to the beneficial bacteria.

Using this method removes the need for adding an ammonia supplement to the tank.

Pros and Cons of Each Method

Cycling without fish means you don’t worry about exposing your betta to toxic ammonia as the nitrogen cycle progresses.

On the downside, fishless cycling can take up to six weeks or more.

Cycling with fish is much faster, and you don’t need ammonia supplements for the process.

But, prolonged exposure to a high ammonia concentration can kill your betta.

Ethical Considerations of Fish-In Cycling

Some fish keepers claim fish-in-cycling is safe for the fish. But considering the harmful effects, I disagree.

Exposing your fish to ammonia for a prolonged period is painful for them.

Ammonia in any amount is toxic for bettas and other fish species.

Prolonged exposure to ammonia and other dangerous toxins can have lasting health effects on your betta.

There is no way of avoiding this exposure in a fish-in-cycle.

Planning for a fishless cycle before you get your betta is the best option.

Keep Your Nitrogen Cycle on the Right Path

Understanding the nitrogen cycle is important in keeping your betta healthy.

Patience, water testing, and more patience help you get through the process.

But in the end, it is all worth creating a healthy and stable environment for your betta.

Learn more about the ideal water parameters for betta fish with our helpful guide at the link.