Cycling an Aquarium for Aquatic Animals

beautiful aquarium

By Ed Kowalski

The nitrogen cycle

Many aquatic animals secrete nitrogenous waste products in the form of ammonia. Furthermore, uneaten food and other wastes produce ammonia as they decompose. In a healthy aquarium, bacteria then feed upon the ammonia and turn it into nitrite (actually HNO2, nitrous acid). Then a second group of bacteria turns the nitrite to nitrate (actually HNO3, nitric acid). This process is depicted in the cartoon below. Ammonia and nitrite are toxic to aquatic animals, and this is where the main problems with new aquariums occur. The term cycling refers to the establishment of an active "biological filter" (or "biofilter") consisting of beneficial bacteria that break down nitrogenous aquatic animal wastes. In a brand new (un-cycled) aquarium, the levels of ammonia and nitrite will rise, and these toxic compounds can reach lethal levels. Once the bacterial populations become established, they break down these compounds into non-toxic forms, and the tank is then considered to be "cycled". It can take anywhere from several weeks to a couple of months to cycle an aquarium.

Cycling with animals

cartoon depicting aquarium nitrogen cycle

The majority of established aquariums were cycled with fish or other aquatic organisms. While this is an effective way to cycle a tank, it can be lethal to the animals unless conducted properly. Some hobbyists consider this method cruel, as it exposes the animals to ammonia.

When cycling with animals, it is important to under-stock the tank with animals, feed them as little as possible, and remove uneaten food and other waste promptly. Care must be taken to test the water daily for ammonia and nitrite. If dangerous levels are found, large scale water changes are needed to avoid sickness or death of the animals.

Fishless (or "amphibianless") cycling

As an alternative, the tank can be cycled without any animals in the aquarium. For this process, you will need test kits for ammonia, pH, nitrite and nitrate, and a source of ammonia. Possible sources for the ammonia are easily decomposed materials such as frozen brine shrimp.

Set up the aquarium using dechlorinated water and let it run for 24 to 48 hours. This will allow you to ascertain that all the equipment is working properly and will allow any excess dissolved gases to dissipate. Test the water for all of the above listed parameters and record the results; these will be your starting measurements. After the aquarium has been running for the recommended time period, add a small amount of your ammonia source. (For example, use a piece of frozen brine shrimp that could cover a dime). After 24 hours, test the water and record the results. Monitor the water results every day or two. When the ammonia level begins to drop and the nitrite levels to rise, add another small piece of ammonia source to keep feeding the bacteria. Keep monitoring the water quality, and when the nitrates begin to rise again dose the tank with the ammonia source. This should be the last dose required to finish the cycle.

At this point, you should see a small rise in the ammonia level, a small rise in the nitrite level and a continuation in the rise of the nitrate level. When the ammonia drops to zero, the nitrite drops to zero and the nitrate is readable the aquarium can be considered cycled. However before you add animals to the aquarium, do a 20% water change to lower the level of the nitrates. Different animals tolerate nitrates to different degrees. As a safety precaution, try to maintain the nitrate levels below 60 ppm through water changes. Usually 10 to 15% water changes each week are sufficient to keep the levels down.

Now the tank is ready for animals. However, the bacterial population will need to grow to match the number of animals added to the aquarium. The easiest way to accomplish this is by adding animals gradually. If you add several animals every 2 to 3 weeks, you should be able to safely increase the number of animals in the aquarium. The water should be tested on at least a weekly basis to monitor water quality and hopefully prevent potential ammonia and nitrite poisoning. Any use of an antibiotic in the aquarium will cause the biological filter to crash and require the tank to be re-cycled. As a final warning, pay close attention to the pH in long-term aquariums, as the pH tends to drop over time in old aquariums (the nitrification process is an acidic process). If the pH drops too low, the biological filter will crash. This can be prevented through regular partial water changes and by using a gravel siphon to clean dirt from the sand or gravel.

Use of household ammonia for fishless cycling

Some hobbyists have used household ammonia to provide the ammonia source for cycling a new tank. Information on this method can be found on various websites. However, this method should be used with caution. Household ammonia is very strong, so only very small amounts (drops) are needed. Any ammonia product containing detergent or other ingredients should obviously be avoided. The pH of the tank must be monitored, as ammonia can cause serious shifts in pH, especially in soft water.

Speeding up the cycling process

The cycling process can be sped up if decorations or a filter from an already established tank is added to the new aquarium, thus seeding the tank with a population of bacterial biofilm. Beneficial bacteria usually cling to solid surfaces inside a tank, so the transfer of water from an old tank will be less effective than the transfer of substrate, ornaments, or filter media.

Heavily planted tanks are also likely to cycle more quickly. Live plants, once established, will consume some nitrogenous waste products. Recent evidence suggests that live plants can directly consume ammonia and nitrite, thus decreasing the toxicity of these compounds during the cycling process. Live plants are also likely to bring beneficial bacteria with them.

The use of bacterial products to speed cycling of a new tank is of questionable value. While these products cause no harm, most of them contain insufficient live bacteria or incorrect species of bacteria.


Conklin, D., Elkins, B., Shelton, K. 2001, The Water Column, Animal Keepersí Forum, 28(8): 308-310.

Riehl, Rüdiger, Baensch, Hans A., 1994, Aquarium Atlas, Mergus, Melle.

Tullock, John, 1997, Natural Reef Aquariums, Microcosm, Shelburne.

Walstad, Diana. Plants and Biological Filtration [online], accessed November 30, 2009.

© 2001 Ed Kowalski. Revised December 2005, Ed Kowalski and Jennifer Macke. Revised November 2009 and August 2011, Jennifer Macke.

Further Reading

Dornhoffer, T. Nitrogen Cycling Revisited: Sand, critters, carbon, and why you may be under-feeding your tank.


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