Cycling an Aquarium Fishless vs Fish-in Cycle

Introduction to Aquarium Cycling

Embarking on the journey of an aquarium hobbyist begins with understanding the essential process of tank cycling. This fundamental step is not merely a recommendation but a requirement for fostering a thriving underwater world. Tank cycling is the first chapter in the life of an aquarium, setting the stage for a balanced, healthy environment where aquatic life can flourish. It's a natural process that prepares the tank for its future inhabitants by establishing a robust biological filtration system.

nitrogen cycle

What is Cycling a Fish Tank?

So, what is cycling a fish tank? In simple terms, it involves the cultivation of beneficial bacteria that are crucial to breaking down toxic ammonia and nitrites, byproducts of fish waste and uneaten food. These bacteria convert harmful substances into less harmful nitrates, which are then removed through regular water changes and plant uptake. Without this vital cycle, aquarium inhabitants would be subjected to toxic conditions, making the understanding and management of this process indispensable for any successful aquarist.

The First Steps in Cycling a New Fish Tank

Cycling a new fish tank is a patient aquarist's game. It starts with setting up your aquarium with all the necessary equipment and filling it with water. The next step is to introduce a source of ammonia to simulate the conditions that will occur when the tank hosts life. There are several ways to do this, and one traditional method is the fish in cycle, where hardy fish are introduced to start the ammonia production. However, this method can be stressful and harmful to the fish, leading many to prefer fishless cycling methods that are safer for aquatic creatures.

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Understanding Fish Tank Cycling: The Basics

The core of a healthy aquarium lies in the nitrogen cycle, a natural process that every tank must go through before it can safely house aquatic life. This cycle is facilitated by nitrifying bacteria, microscopic heroes that convert harmful ammonia produced from fish waste and excess fish food into less toxic substances.

nitrogen cycle

The presence of these bacteria is a sign of a maturing aquarium. The cycle begins with ammonia being converted into nitrites, both of which are harmful to fish at high ammonia levels. Then, different beneficial bacteria convert the nitrites into nitrates, which are less harmful and can be absorbed by plants or removed through water changes.

Methods of Cycling Aquarium: Fishless vs. Fish-in

There are two primary methods to cycle an aquarium: the fishless cycle and the fish in method. Fishless cycling is an increasingly popular method that involves adding ammonia to the new tank without the presence of fish, hence the name. This method allows beneficial bacteria to grow and start the nitrogen cycle without putting any fish at risk. The fish in method, although traditional, involves adding a few hardy fish to the tank to provide the initial source of ammonia. However, this method requires diligent monitoring to ensure ammonia and nitrite levels don't rise to levels dangerous for the fish.

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The Fish-In Cycle: Traditional but Controversial

The fish in cycling method has been used for decades, allowing the nitrogen cycle to initiate through the introduction of fish and the subsequent production of fish waste. While this method can be quicker in establishing a cycled aquarium, it is not without its risks. The ammonia and nitrite levels can spike quickly and without much warning, potentially leading to stressful and even fatal conditions for the fish. This has made fish in cycling a controversial choice among aquarists who prioritize the welfare of their aquatic pets.

Pros and Cons of Fish-In Cycling

The fish in cycling method can offer immediate gratification for eager aquarium owners looking to see life in their new tank. However, this method's major drawback is the risk of high ammonia levels and nitrite, which can be toxic to the fish. On the plus side, if managed carefully, this method can establish a cycled aquarium relatively quickly, with nitrifying bacteria benefiting directly from the continuous supply of fish waste. But it requires constant water testing and possible intervention to keep the ammonia and nitrite at safe levels, which can be labor-intensive and stressful for both fish and aquarist.

The Fishless Cycle: A Safe Alternative

In contrast, the fishless cycle represents a humane and increasingly preferred method among aquarists. This approach eliminates the ethical concerns of exposing fish to potentially harmful conditions. By adding a pure ammonia source to the new tank, aquarists can precisely control ammonia levels, providing an ideal environment for beneficial bacteria to establish themselves. Fishless cycling requires patience, as the process can take several weeks, but it ensures that the aquarium is fully safe for its future inhabitants. Additionally, the absence of fish during this critical period removes the risk of mortality due to ammonia and nitrite toxicity.

Step-by-Step Guide to Fishless Cycling

The process of fishless cycling involves several clear steps to ensure success. Firstly, prepare the new tank with all the necessary equipment and conditions suitable for the beneficial bacteria to thrive.


Add ammonia to the tank to initiate the cycle, closely mimicking the nitrogen cycle that occurs naturally in an established tank. Monitor the ammonia levels and nitrites regularly using test kits to track the progress of the cycle. Once these levels start to drop and eventually reach zero, it’s an indication that nitrifying bacteria are effectively converting them into nitrates. At this point, the tank is considered cycled, and it's safe to introduce fish into their new, balanced habitat.

Monitoring Ammonia and Nitrite Levels During Cycling

During the critical period of establishing a new aquarium, it's essential to diligently monitor ammonia and nitrite levels. These toxic compounds are the byproducts of organic decay from fish waste and uneaten fish food. Regular testing allows for the tracking of the nitrogen cycle's progression, ensuring that nitrifying bacteria have adequately colonized the tank and are efficiently converting harmful ammonia into less harmful nitrates. Testing kits, which are readily available and easy to use, should become a staple tool for any responsible aquarist during this phase, as maintaining safe levels of these compounds is crucial for creating a healthy environment for future aquatic life.

The Role of Fish Food and Waste in Aquarium Cycling

Even without fish present, fish food can be used in a fishless cycle as a source of ammonia. As the food breaks down, it mimics the fish waste that would naturally produce ammonia in an inhabited tank. This method is an alternative to direct ammonia dosing and can be more accessible for beginners. However, it requires careful management to avoid overfeeding and creating an excess of ammonia. Whether using fish food or pure ammonia, the goal is to feed the developing nitrifying bacteria and encourage their multiplication, which is the cornerstone of a successful cycle.

Nurturing Beneficial Bacteria: The Heart of the Cycle

Beneficial bacteria are the unsung heroes of the nitrogen cycle, quietly working to transform a tank full of toxins into a safe home for aquatic creatures. These bacteria come in two main types: those that convert ammonia into nitrite, and those that convert nitrite into nitrate. Cultivating a robust colony of nitrifying bacteria is key to a healthy aquarium. This can be encouraged by maintaining a stable environment with the right temperature and pH levels, and by providing an uninterrupted supply of ammonia, either from fish waste in a fish in cycle or from dosing in a fishless cycling process. Once established, these bacteria will serve as the primary filtration system, keeping ammonia levels and nitrites in check.

Achieving a Cycled Aquarium: Knowing When It's Ready

Understanding when an aquarium has successfully completed the cycle is crucial for the health of its future inhabitants. An aquarium is considered cycled when tests consistently show no detectable ammonia or nitrite, and only nitrates are present, indicating that nitrifying bacteria have established well enough to process waste efficiently. It is a signal that the new tank has achieved a biological balance. At this juncture, it's vital to perform a large water change to reduce the accumulated nitrates to safe levels. This step marks the transition from a sterile environment to a cycled aquarium ready to support life, providing a safe and stable home for aquatic species.

Adding Fish to a New Tank: Timing and Techniques

The transition from cycling to introducing fish to a new tank should be gradual and thoughtful. Once the tank is cycled, adding fish slowly, over several weeks, allows the beneficial bacteria populations to adjust to the increased bioload. Start with a few hardy species that are known for their ability to adapt to new environments.


Monitor ammonia levels and nitrites closely after adding each group of fish, as an influx of fish waste could cause a mini-cycle if the bacteria are not yet sufficiently established. Proper acclimation techniques, such as temperature matching and slow introduction of tank water to the fish's bag, are crucial to reduce stress and prevent shock, ensuring a smooth transition for the new inhabitants.

The Added Layer of CO2 Injection in Aquarium Cycling

Introducing CO2 into an aquarium can be a game-changer, especially in planted tanks where the growth and health of aquatic plants are a priority. However, CO2 injection adds another layer to consider during the cycling process. While CO2 doesn't directly affect the nitrogen cycle, it can lower the pH of the water, which in turn can influence the activity of nitrifying bacteria. It's crucial to keep an eye on pH levels when cycling with CO2 injection, as extreme changes could inhibit bacterial efficiency and stability. Regular testing becomes even more imperative to ensure the cycling process continues smoothly and the ammonia levels remain in check.

Maintaining a Healthy Aquarium Post-Cycle

After the initial cycle, maintaining the equilibrium of your aquarium is an ongoing commitment. The post-cycle period involves regular monitoring of ammonia levels and nitrite to ensure they remain at zero, while nitrates should be kept at manageable levels through routine water changes. It's also essential to continue monitoring the health of the beneficial bacteria colonies, as they are the backbone of the nitrogen cycle in your tank. Proper feeding, avoiding overstocking, and regular maintenance such as cleaning filters and substrate will keep these bacterial colonies thriving. Additionally, keeping a close eye on the fish for any signs of stress or disease can help catch any potential issues early.

Cycling an Aquarium with CO2 Injection

Cycling a tank with CO2 injection requires a delicate balance. While CO2 enrichment is beneficial for plant growth, its impact on pH levels can influence the cycling process. It's important to maintain a stable pH when injecting CO2 to ensure optimal conditions for nitrifying bacteria.

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Advanced aquarists often use CO2 injection in conjunction with careful monitoring of carbonate hardness (KH) to buffer against pH swings. The key is to increase CO2 levels gradually and to keep testing water parameters frequently to ensure the health of both the beneficial bacteria and the plants that rely on the CO2 for photosynthesis.

Conclusion: Choosing Between Fishless and Fish-In Cycling

In conclusion, whether to opt for a fishless cycle or a fish in approach is a decision that should be made with both the health of the aquatic life and the stability of the ecosystem in mind. A fishless cycle is often recommended for its safety and humane approach to establishing nitrifying bacteria without risking the well-being of fish. Meanwhile, the fish in method, though traditional, requires vigilant management to safeguard against the toxicity of ammonia and nitrite. Regardless of the method chosen, the success of an aquarium starts with a patient and informed cycling process, laying the foundation for a thriving aquatic environment for years to come.

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