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Cycling an Aquarium: Why It Matters and How to Do It

April 13, 2019 5 min read

What is cycling?

Cycling refers to the process of establishing a population of living bacteria within an aquarium. Housed on the filter media, substrate, and surfaces inside the aquarium, bacteria biologically filter the water as they carry out their normal life functions. Collectively referred to as a biofilter, the bacterial colonies should be well-established before introducing animals into the aquarium, or survival is unlikely.

How long does cycling an aquarium take?

There is no magic number, but most aquariums will take anywhere from a few weeks to a few months to cycle. The ability of the biofilter to sustain an aquarium depends on a number of factors and the only way to be sure is to test the water and understand what to look for. 

What does the biofilter do?

As water passes through the biofilter, bacteria consume toxins that have been dissolved into the water from the decomposition of normal organic matter (fish waste, uneaten food). Without the bacteria, toxic ammonia would build up in the water, which deteriorates the tissue of fish, coral, and invertebrates. Exposure to ammonia can be fatal in a matter of hours. 

How does a biofilter get established?

Once an aquarium is filled with de-chlorinated water, bacteria will find their way into the aquarium from the environment, and will thrive if there is a source of ammonia to feed on. A pinch of fish food every day should effectively release ammonia to nourish a developing bacterial colony. Alternatively, a substrate of fresh aqua soil should achieve the same result. Some sources recommend using filter media from another established tank (such as from your local fish store) or live rock to introduce pre-established bacteria. While this could help speed up the cycle, the potential for introducing contaminants into the aquarium should not be ignored. Bottled bacteria is contaminant-free and can be added as a safer method to speed up the cycling process. 

When is the aquarium safe for fish?

The most definitive way to know if the biofilter is ready is by regularly testing during the cycling process. Specifically, the compounds ammonia, nitrite, and nitrate are at play. Data should reveal an ammonia spike, followed by a nitrite spike, followed by a sustained nitrate spike (more on that later). When test results reveal that ammonia and nitrite concentrations have reached zero, the aquarium is safe for fish. Note that a common mistake is to wait a month, test the water, find zero ammonia and nitrite, and assume the aquarium is ready when actually it may never have cycled in the first place.

Bottled Bacteria

Bottled bacteria and other inoculated products such as live sand can be used to accelerate the cycling process to as little as a matter of days by introducing all types of essential bacteria at the same time. As a result, the ammonia and nitrite spikes many never occur or be detected because the compounds don't have an opportunity to accumulate. Unless an aquarium monitor is being used to give continuous readings, a more practical testing approach following a bacteria dose is to add a bioload (e.g. fish food), and assume the aquarium is cycled if test results show zero ammonia, zero nitrite, and non-zero nitrate after a matter of hours. 

Adding Fish, Invertebrates, or Coral

Newly cycled aquariums are vulnerable to ammonia spikes and should be stocked slowly and incrementally. When the first fish is added, this is a drastic increase to the bioload. While the biofilter might have successfully processed whatever amount of ammonia was present during cycling, a live fish constantly eating and excreting is usually much more than the young biofilter is used to. We recommend adding one or two fish at a time to allow the bacterial colonies to adapt to the new bioload. If testing results show zero ammonia and nitrite in the few days after adding fish, then it’s usually safe to add a few more. It's all too common for new aquarists to add too many fish at once and witness a fatal build-up of ammonia as the influx of nutrients to the system can’t be processed fast enough by the biofilter.

What’s the Chemistry behind all this?

There are 3 compounds to be aware of: ammonia, nitrite, and nitrate. The ammonia-consuming bacteria release nitrite as part of their biological functioning, and another type of bacteria consume the nitrite, releasing nitrate. This is sometimes referred to as the nitrogen cycle, although it really represents only a part of the cycle.

Ammonia and nitrite are both toxic to fish at any concentration and should not be present in an established aquarium. Low levels of nitrate (an important plant nutrient) are safe and even beneficial in reef and planted aquariums, but higher concentrations can be deadly for aquatic animals. 

While an aquarium is cycling, ammonia should spike at first (as organics decompose) and then decline as the bacteria develop to consume it. Nitrite should then spike as the bacteria’s waste accumulates, and then decline as the next bacteria develop to consume it. Nitrate should then build up, and will continue to increase in concentration until it is manually removed from the aquarium by changing the water. 

Once both types of bacterial colonies (the ammonia consumers and nitrite consumers) develop, and are able to process ammonia and nitrite as they are produced, the biofilter is established and the tank is cycled. This means that even when organic matter is introduced, and then decomposes, ammonia should never show a reading because hungry bacteria consume it immediately.Cycling Chart

The Nitrogen Cycle

The nitrogen cycle is often cited as a prerequisite to understanding aquarium cycling. Nitrogen atoms are passed from food to waste, and then from ammonia to nitrite to nitrate. Plants consume nitrate, which is then eaten by animals, and the nitrogen atoms start the process over again. This takes place in natural ecosystems, but in most aquariums, the nitrogen cycle is completed by a human who introduces nitrogen to the system by feeding and removes it during a water change. 

Are water changes necessary during cycling?
While not essential, we recommend water changes during cycling, although opinions differ. Since bacteria live on surfaces, removing water does not disrupt their development. Water changes can help control the amount of ammonia in the first stage of the aquarium’s life. High ammonia can be beneficial for aquarium plants, but algae is also a plant, and indeed, unwanted algae blooms are common during cycling. Our mindset is that it can't hurt to immediately get into the habit of this essential maintenance task and get the ecosystem used to having regular water changes, which remove excess nutrients and replace depleted minerals. 


Keep the biofilter alive

Maintaining the biofilter is essential throughout the life of the aquarium. Follow our general cleaning guide to make sure you don't accidentally kill your biofilter (hint: never let any component of your aquarium touch un-treated tap water).



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