How to Establish the Nitrogen Cycle in an Aquarium

The nitrogen cycle (also known as the nitrification process) refers to a method of breaking down harmful nitrogen waste in the aquarium into less toxic substances. To initiate the nitrogen cycle, beneficial bacteria that consume the waste need to be cultivated in the aquarium's filtration system. Introducing fish into a tank that hasn't completed the nitrogen cycle is strongly discouraged – the chemicals in the waste can stress the fish significantly and may even cause their death. Therefore, anyone setting up a new aquarium must establish the nitrogen cycle to ensure the well-being and safety of the fish.

Set up the aquarium and filtration system. To begin establishing the nitrogen cycle, you need to fully set up the aquarium and place everything you want along with the fish. You can refer to our article on setting up freshwater and saltwater aquariums for additional guidance. Below is a list of the tasks you need to complete before starting; this list may not apply to every type of aquarium:

• Set up the aquarium
• Add substrate material
• Fill the tank with water
• Install air stones, pumps, etc.
• Add plants, rocks, etc.
• Install the filtration system (and/or protein skimmer)
• Set up the heater

Introduce a few easy-to-care-for fish into the tank. The purpose of the nitrogen cycle is to introduce fish into the tank to produce waste, but the fish must be able to survive in an environment with high toxin levels long enough for the bacteria to process the waste. Therefore, you should choose fish species that are hardy and capable of generating good bacteria, and start with a small number. As the bacteria grow, you can gradually introduce more fish. Here are some suitable species:

• White Clouds
• Zebra Danios
• Tiger Barbs
• Pseudotrophius Zebras
• Banded Gouramis
• X-ray Tetras
• Pupfish
• Most minnows
• Most guppies

Feed the fish sparingly. When setting up the nitrogen cycle in the aquarium, it is important not to overfeed the fish. While different fish species have different nutritional needs, the general rule is to feed the fish every other day with a small amount; do not leave uneaten food behind. There are two main reasons for this:

• Overfeeding causes more waste, raising the toxin levels in the tank before the bacteria can stabilize them.
• Uneaten food will decompose and also produce toxins.

Change the water frequently. While waiting for the nitrogen cycle to establish, you should change about 10-25% of the water in the tank every few days. Similar to the feeding schedule, reducing the water changes is another way to prevent toxin levels from rising too high before the bacteria can grow. If you have a saltwater tank, be sure to add the correct amount of marine salt during each water change to maintain proper salinity.

• Do not use chlorinated water, as chlorine can kill the bacteria, and the nitrogen cycle will need to start over. If using tap water, treat it with a dechlorinator or appropriate water conditioner before adding it to the tank. If using bottled water, make sure it's distilled water, as "purified" or "drinking" water may contain minerals that are harmful to fish.
• Change the water more frequently if you notice signs of fish stress due to ammonia (see more information in the "Handling Common Issues" section below). However, try to avoid stressing the fish by preventing large fluctuations in water temperature or chemicals.

Use a testing kit to monitor toxin levels. Once the fish are added, levels of toxic chemicals like ammonia and nitrites will rise quickly in the water. As the beneficial bacteria begin to develop and react with these chemicals, toxin levels will drop back to near "zero" – a safe level for adding more fish. You can monitor these chemicals using a test kit, which is commonly available at aquarium stores. Ideally, test the water every day, but sometimes you can also test every few days.

• You should keep ammonia levels below 0.5 mg/L and nitrites below 1 mg/L throughout the cycle (ideally, these levels should be under half of those values). If these chemicals start to rise to unsafe levels, you should increase the frequency of water changes.
• The nitrogen cycle will be complete when both ammonia and nitrites drop to undetectable levels. In practice, this is often considered to be the "zero" level, though technically it may not be completely accurate.
• You can also take a water sample to the store where you purchased your fish or aquarium. Most of these places offer water testing services for a low fee (or even for free!).

Gradually introduce more fish as toxin levels drop near "zero". The nitrogen cycle typically takes about six to eight weeks to complete. You can start adding more fish when ammonia and nitrite levels drop to undetectable levels with a test kit. However, you should do this gradually, adding only one or two fish at a time. Introducing fish slowly ensures that the levels of ammonia and nitrites only rise to a level that the bacteria can handle.

• After each addition, wait at least a week and test the water again. If ammonia and nitrites are still low, you can add more fish.

Set up and prepare the aquarium. For this method, you'll begin with a fully set up aquarium, similar to the previous method, but this time no fish are added until the entire nitrogen cycle is completed. Instead of introducing fish, you'll add organic waste, monitor the water levels, and wait for the cycle to complete.

• Patience is key, as this method requires you to wait for the organic matter in the tank to decompose and begin producing toxic waste. However, this is often seen as the more "humane" choice, as it avoids exposing fish to ammonia and nitrites like in the previous method.

Sprinkle some fish food into the tank. To kickstart the nitrogen cycle, add a small amount of fish food flakes, just like you would normally feed your fish. After this, all you have to do is wait. A few days later, the food will begin to decompose, releasing toxins (including ammonia) into the water.

Test ammonia levels after a few days. Use a test kit (or take a water sample to a local aquarium store) to measure ammonia levels. The ammonia concentration should reach at least three parts per million (ppm). If the ammonia level is not high enough, add more fish food and wait for it to decompose before testing again.

Aim to maintain ammonia levels around 3 ppm. Continue testing the ammonia levels every two days. As the beneficial bacteria begin to grow in the tank, they will start consuming the ammonia, helping to reduce its concentration in the water. Add more fish food whenever the ammonia level falls below 3 ppm to maintain the cycle.

Begin testing nitrite levels after one week. As the bacteria begin consuming ammonia, they will start releasing nitrites, an intermediate chemical in the nitrification process (less toxic than ammonia, but still harmful to fish). Start testing for nitrites after one week; as with ammonia testing, you can use a test kit or take a water sample to an aquarium store.

• When nitrites are detected in the water, the cycle has begun. At this point, you should continue to increase ammonia levels as before.

Wait for nitrites to drop suddenly and nitrates to rise. As you continue feeding bacteria in the tank with ammonia, nitrite levels will continue to rise. However, over time, beneficial bacteria will grow enough to convert nitrites into nitrates, the final chemical form in the nitrification cycle (which is harmless to fish). When this happens, you'll know the cycle is nearing completion.

• You can recognize this final stage by testing for nitrites (where you'll see a sudden drop in nitrites), nitrates (look for nitrates spiking from zero), or both.

Gradually add fish to the tank once ammonia and nitrite levels are close to "zero". After six to eight weeks, ammonia and nitrite levels will drop to undetectable levels, while nitrate levels will stabilize. It is now safe to introduce fish.

• However, just like the previous method, introduce fish gradually. Only add a few at a time and wait at least one or two weeks before adding more.
• Consider cleaning the substrate with a siphon before adding more fish, especially if you’ve been feeding a lot. Decomposing food or organic matter can become a time bomb. If organic debris gets trapped beneath the gravel, ammonia won’t leak into the water, but if disturbed, a large amount of ammonia can quickly escape.

Use filter material from an established tank to speed up the cycle. Since creating a nitrogen cycle can take 6 to 8 weeks, many aquarium owners look for ways to shorten the process. One widely recommended method is to introduce bacteria from an established tank into a new one. By skipping the wait for bacteria to grow naturally, your tank will complete the cycle more quickly. A good source of bacteria is the aquarium filter; you can simply transfer the filter material from an established tank to the new tank to speed up the process.

• Try to find filter material from a tank with similar size and fish quantity. Using a filter from a tank with fewer fish for a tank with more fish may result in ammonia buildup outpacing the bacteria's ability to process it.

Supplement with gravel from an established tank. Just as filter material can help you "seed" beneficial bacteria from an established aquarium to a new one, the substrate (gravel) from a tank that has completed the nitrogen cycle can have a similar effect. Simply sprinkle a handful of gravel on top of the substrate in the new tank.

Plant aquatic plants in the aquarium. Live aquatic plants (as opposed to plastic ones) can significantly accelerate the nitrogen cycle, especially if they come from an established tank. Aquatic plants not only carry beneficial bacteria (similar to the substrate mentioned above), but they also directly absorb ammonia through a biological process known as protein synthesis.

• Fast-growing plants (such as Vallisneria and Hygrophila) are typically the best at absorbing ammonia. Floating plants on the water's surface also work well.

Be cautious of cross-contamination. A downside to transferring filter media or substrate material to introduce beneficial bacteria from one tank to another is the potential to inadvertently introduce other organisms. Many species of parasites, invertebrates, and mixed microorganisms can spread in this way, so be aware of this risk and never use materials from a tank that may harbor harmful organisms.

• Pest species that can be transferred this way include snails, harmful algae, and parasites like ich and velvet fungus.

Add a small amount of salt to a freshwater tank. For freshwater aquariums, you can add a pinch of salt to help keep the fish healthy when toxins are at their peak at the beginning of the nitrogen cycle. This can help reduce the toxicity of nitrites, an intermediate chemical in the nitrification process. However, you should not exceed about 12 grams of salt per 4 liters of water. Higher amounts can cause significant stress to freshwater fish.

• Make sure to use aquarium salt, as table salt is not suitable for fish tanks and can harm the fish.

Mitigate ammonia stress in the nitrogen cycle by frequently changing the water. Ammonia stress (a dangerous symptom in fish that occurs when ammonia levels rise too high) is always a risk during the nitrogen cycle. If not addressed promptly, these symptoms can eventually lead to fish death. If you observe any of the following signs in your fish, reduce ammonia levels by increasing the frequency and volume of water changes:

• Sluggish or minimal movement (even when food is added to the tank)
• Refusal to leave the tank's bottom
• Gasping for air at the water's surface
• Eye, gill, and/or anus inflammation

Consider using ammonia neutralizing agents if toxin-related issues arise. These products come in two types: ammonia removers and detoxifiers. Most pet stores offer specialized chemicals to remove ammonia from aquariums. These can be helpful when ammonia levels become harmful to the fish, but they are even more effective when setting up a new tank. By using these products, you can skip a few water changes and accelerate the nitrogen cycle in the new aquarium.

• Some people believe ammonia removers can be harmful in the long term. This misconception may arise from misunderstanding the detoxification process. Toxic ammonia (NH3) is in a state of equilibrium with less toxic ammonium ions (NH4+). Most detoxifiers work by converting toxic ammonia into the non-harmful ammonium form. However, ammonia will re-evaporate after 24-48 hours. This is why these products should be used as follows:
  • Continue use until beneficial bacteria stabilize, AND
  • Occasionally use them during partial water changes (as directed by the manufacturer) to remove some of the accumulated ammonia, AND
  • Even if not explicitly stated, you should dose the detoxifier for the entire tank, not just the new water, as ammonia may resurface from the tank itself (24-48 hours after the previous dose).
• When performing a 50% water change (or more), the time required for the nitrogen cycle to complete will extend (or even stop entirely) due to the temporary suppression of beneficial bacteria. These bacteria will need time to adjust to the new pH level. Therefore, some suggest that pH should not change by more than 0.2-0.3 per day. For instance, if your tank's pH is 7.8, a 25% water change with pH 7 water would result in a final pH of 7.6.
• Beneficial bacteria only convert ammonium ions (non-toxic), so these products are also beneficial to bacteria.

Only use goldfish to cycle a tank intended solely for goldfish. While often considered a classic aquarium fish, goldfish are not recommended for cycling tanks. This is because goldfish have different care needs compared to the more common tropical fish. Using goldfish to cycle a tank and then adding tropical fish may cause some bacteria to die off due to the higher temperatures and differing water conditions. This can create stress for the goldfish, the bacteria, and the tropical fish. Therefore, this is not an ideal method for maintaining a healthy aquarium environment.

• Additionally, new goldfish can easily contract diseases that could spread to the rest of the tank.
• You should avoid cycling any tank with a type of goldfish known as "feeder" goldfish, which are typically poorly cared for by breeders and sellers, making them more susceptible to illness.

• Pure ammonia can also be used to cycle a tank without fish. Only use pure ammonia with no additives. You can calculate the proper dosage by searching for "ammonia dosage chart" online.
• Don't hesitate to consult an expert if you have questions about your aquarium. Better safe than sorry! However, be aware that many pet stores do not employ aquarium specialists.
• Another method to speed up the nitrogen cycle is to use bacterial supplements. Most pet stores sell cultured bacteria, so if you're willing to spend a bit more, you won’t have to wait six weeks to complete the nitrogen cycle. However, some people feel that the bacteria in these products aren't effective, so it's still best to "test" the bacteria with ammonia.

• If nitrate levels exceed 40 ppm and ammonia/nitrite levels surpass 4 ppm, it’s time to make slight adjustments to the water, as these conditions may harm the healthy bacteria you are cultivating.
• Using large pieces of food or organic matter to cycle the tank (which releases ammonia) can lead to excessive bacterial growth and unpleasant odors. The food may also mold underwater, potentially causing disease in the fish and creating a breeding ground for mold in the substrate.