How to Cycle an Aquarium

Have you ever wondered why you have to cycle your aquarium before you can add fish and corals?

The reason you must cycle an aquarium is because it allows time for beneficial bacteria to grow inside your fish tank. Once these bacteria exist, a unique cycle begins which creates a more stable environment for fish and corals to safely inhabit.

In fact, this cycle is the sole reason we are able to bring a little slice of the ocean into our homes. The Aquarium Nitrogen Cycle is often referred to as biological filtration within the hobby. This short article and the accompanying diagram explain how the Nitrogen Cycle for an aquarium works and why patience is one of the keys to preventing “New Tank Syndrome”—Ammonia and Nitrite poisoning.

Nitrogen Cycle

Ammonia is introduced into your aquarium water from fish waste, leftover fish food and dead or decaying plants and animals. Ammonia is extremely toxic to fish, but can quickly kill just about any aquatic life inside an aquarium. In other words, it must be removed!

This brings us to the next step in the process. The bacteria mentioned earlier come into play in this stage. Nitrosomonas bacteria absorb Ammonia from your aquarium water and then release it as Nitrite via a metabolic process within the bacteria. While not as toxic as Ammonia, Nitrite is still harmful to your aquatic friends so it must also be removed.

Fortunately, Mother Nature has a solution. Bacteria called Nitrobacter will begin to develop that will absorb the Nitrite and release it back into your aquarium as Nitrate.

Nitrate is the final by-product of the nitrogen cycle in an aquarium and is far less toxic to animals compared to Ammonia and nitrite. But Nitrate must also be removed from your tank water. Left alone, Nitrate will build up and, over time, lead to a number of problems in freshwater and saltwater aquariums alike.

The easiest and most effective way to remove Nitrate is physical removal. Performing a Water Change with clean, Nitrate-free water will dilute the existing Nitrate in your aquarium to a level that is safe for livestock.

Another way Nitrate is removed from an aquarium is by Anaerobic Bacteria. Anaerobic Bacteria are found in your aquarium’s cycled rock and deep within the substrate—areas with lower oxygen levels. Anaerobic Bacteria synthesize Nitrate and release it as a mixture of gases into the atmosphere. This is why you may see what appear to be pockets of air in a deep sand bed.

Anaerobic Bacteria can quickly dispel Nitrate, but relying on the bacteria as the sole approach to removing Nitrate is unreliable. The results may be unstable and the mixture of gases released can become toxic to fish, coral and invertebrate once dissolved in tank water. This is actually a common problem with deep sand beds.

Plants and Macroalgae also absorb Nitrate. That is why many reef aquarium owners will have a refugium full of Macroalgae and/or mangroves. Macroalgae absorbs Nitrate from aquarium water and uses it like a fertilizer. This method of removal is effective, although it does not replace the need for regular partial Water Changes. Recurrent Water Changes, plenty of Anaerobic Bacteria and a refugium full of Macroalgae are an ideal combination that together offers the best approach to keeping your aquarium water clean.

Understanding the aquarium water cycle makes life as an aquarium keeper much easier. Many of the problems in fish tanks are created when the Nitrogen Cycle in your aquarium is unbalanced. Illness can quickly spread when fish are stressed due to high Nitrate levels. Algae will overcome an aquarium within a matter of days if too much fish waste is introduced. Yellow, stinky water is caused by too much Ammonia. Fortunately, all of this unpleasantness can be avoided and cured by promoting a healthy Nitrogen Cycle in your aquarium.

Please read how to cycle a saltwater aquarium for additional tips and tricks like how to speed up the cycle time and how to know when your aquarium is cycled. You can also use our handy reef tank parameters chart to check ideal water conditions to help when performing water tests and/or dosing additives.