Aquarium Nitrogen Cycle: Everything that You Need to Know


As an aquarium enthusiast, you may have come across the term “Aquarium Nitrogen Cycle,” “Biological Cycle,” or “Startup Cycle.” All of these refer to the same integral process, which is as fascinating as it sounds. The Aquarium Nitrogen Cycle is fundamental to establishing a healthy aquarium ecosystem. The whole cycle revolves around a balancing act between your aquatic creatures producing waste and the nitrifying bacteria that effectively recycle this waste.

The breakout begins with nitrogenous waste, primarily in the form of ammonia (NH3), that proceeds into nitrites (NO2-) and then finally transforms into nitrates (NO3-). While ammonia and nitrites are toxic, nitrates, on the other hand, are relatively safe for your aquatic pets, but only in small quantities. As a vital aspect of aquarium upkeep, understanding the nitrogen cycle is key for healthy and successful fishkeeping.

Importance of Aquarium Nitrogen Cycle

Contemplating the ocean or a river, nature has its own way of dealing with waste. Similarly, in an enclosed ecosystem such as an aquarium, this process mimics nature’s way of waste management. The nitrogen cycle is the cornerstone of this mechanism. Without it, the toxic waste that your beloved fish produce could build up, resulting in detrimental conditions within the aquarium.

Consequently, your fish could fall ill or even die. Specifically, the nitrogen cycle regulates waste accumulation effectively, maintains a steady pH, and prevents abrupt water chemistry changes, ensuring a healthy, stress-free environment for your aquatic pals.

Aquarium Nitrogen Cycle

Understanding Ammonia in the Aquarium

➣ How Ammonia Accumulates

Ammonia can invade your aquarium through various channels. Primarily, it comes directly from the metabolic waste of fish, as they intake proteins and excrete ammonia. Furthermore, unconsumed food, dying plants, and any organic substance left to rot contribute to its accumulation. It’s crucial to note that even tap water can sometimes contain traces of ammonia, making water testing before introduction a wise practice in fishkeeping.

➣ Dangers of Ammonia

Being extremely toxic, ammonia poses significant threats to your aquarium’s life. From delayed growth in juveniles to general vitality loss in mature species and, worst-case scenario, death, it’s evident how detrimental ammonia can be. It affects the fish’s gills directly, making them gasp for air, show decreased activity, or display unusual swimming patterns. It can cause inflammation in the eyes, depicts visible gill discoloration, and produces skin patches, often leading to severe, irreversible damage.

Nitrifying Bacteria: Nitrosomonas and Nitrobacter

Mad respect for these quiet worker bacteria, Nitrosomonas and Nitrobacter, the unsung heroes behind the curtains. Their role is crucial in neutralizing the harmful effects of ammonia. First, Nitrosomonas bacteria perform a process called nitrification, converting toxic ammonia to a less harmful substance, nitrite. Then Nitrobacter bacteria take over, working on transforming nitrite to nitrates, which, although not entirely harmless, are surely less toxic to your aquatic friends compared to the previous forms

You can learn more about Nitrifying Bacteria here.

Maintaining the Aquarium Nitrogen Cycle

Understanding the process of the nitrogen cycle is one thing, but maintaining it requires consistent effort. Once it’s up and running effectively, it’s crucial to maintain the balance to provide a stress-free environment for your fish. Here are some essential tips to preserve the robustness of the nitrogen cycle:

Aquarium Nitrogen Cycle

➣ Regular Monitoring with a Testing Kit

Always keep a testing kit handy. Regular monitoring of ammonia, nitrites, and nitrates is crucial, not just to keep tabs on your aquarium but also to detour any potential misadventures in your aquarium promptly. Ideally, ammonia and nitrite levels should always remain at zero, while the nitrates should be kept as low as possible.

➣ Consistent Water Changes

While useful bacteria are taking care of ammonia and nitrites, nitrates can become the new troublemakers if left unchecked. Scheduled water changes, preferably 20%-25% on a weekly basis, can dilute nitrates concentration and remove other potential toxins that have accumulated over time.

➣ Avoiding Overcrowding in the Tank

Overcrowding is one of the typical rookie mistakes during initial fishkeeping days. Remember: More fish means more bioload. Balancing the number of fish as per the aquarium size allows effective waste management and prevents overloading the biological filtration capacity.

➣ Prudent Feeding of Fish

Feeding your fish may seem like a simple task, but it’s an art to feed them properly. Overfeeding not only leads to obesity-related health issues in fish but also contributes to unchecked waste. Excess uneaten food slowly decomposes, leading to a surge in ammonia levels.

➣ Slow Introduction of New Fish

While welcoming new finned friends to your existing underwater community, remember the impact they’ll bring to your tank’s biological load. Your bacteria need time to adjust to and manage the additional waste. Thus, stock your tank slowly.

➣ Ensuring Good Filtration

Filtration systems do more than just filter out physical debris; they house the beneficial bacteria and provide them with the oxygen they need to work efficiently. A reliable, good-quality filter helps establish a stable cycle.

➣ Utilizing Live Plants

Live plants and algae are not just for show; they play an essential role in the nitrogen cycle. They absorb nitrates for their growth, practically feeding off your aquarium’s waste. Keeping your aquarium planted can assist in the regulation of the nitrogen cycle, contribute to your aquarium’s beauty, and provide additional benefits to your fish, such as shelter and stress relief.

Nitrates in Your Aquarium

➣ The Good and the Bad of Nitrates

Nitrates, the final product of the nitrogen cycle, exert considerably less toxicity than ammonia or nitrite, though still harmful. However, as the famous saying goes, “too much of anything is bad,” a larger quantity of nitrates can also lead to water toxicity. Surprisingly, smaller amounts of nitrates have a beneficial role in the natural ecology of an aquarium, often serving as a food source for live plants and algae, aiding their growth and resilience.

➣ The Cycle of Denitrification

Many believe the nitrogen cycle ends with nitrates, but it doesn’t. In oxygen-limited spaces within the aquarium, such as the inner layers of substrate or inside porous filter media, another type of bacteria comes into play – denitrifying bacteria. In these areas, they convert nitrates into nitrogen gas, which harmlessly escapes into the atmosphere. This process, known as denitrification, closes the loop on the nitrogen cycle and provides a natural outlet to limit the accumulation of nitrates in your aquarium.

Monitoring and Controlling the Nitrogen Cycle

➣ Ways to Test Nitrogen Levels

To ensure the nitrogen cycle runs smoothly in your aquarium, you must monitor the levels of nitrogen compounds regularly for signs of imbalance. A myriad of testing kits are available on the market, explicitly designed to measure ammonia, nitrite, and nitrate levels. Performing these tests weekly can help you keep your finger on the pulse of your aquarium’s health, although as you get more familiar with your aquarium’s nitrogen cycle and how it reacts to changes, you may feel comfortable testing less often.

nitrogen testing kit

➣ Methods for Controlling Nitrate Levels

Nitrate control is the final piece needed to keep your aquarium water safe and healthy. Routinely replacing about 25% of the water in your aquarium can significantly help limit nitrate build-up. Use a high-quality water conditioner during these changes to neutralize any chlorine present in tap water, which can harm your fish and the beneficial bacteria that run the nitrogen cycle.

Similarly, avoid overfeeding, which aids waste build-up and ensures no résiduel food lingers in the aquarium. Regularly vacuuming the aquarium substrate can eliminate accumulated detritus, helping to prevent ammonia from even entering the cycle in the first place. Live aquatic plants or algae can also assist in nitrate control, as they utilize nitrates as a nutrient source.