Easy Guide to Cycling Your Tank
& Preparing for Your New Arrival
What Is Tank Cycling?
Cycling establishes a community of beneficial microorganisms in your tank that turn toxic ammonia into much safer compounds.
Ammonia is produced by fish as a waste product of metabolism and is released into the water. In most aquarium fish, the gills are the main route by which ammonia leaves the body, while urine contributes a relatively small proportion. Faeces, uneaten food and decaying plant material also release ammonia as they break down.
Without an established cycle, ammonia and nitrite can build up quickly, causing serious illness or death — especially in fish already stressed by a postal journey.
A fully cycled aquarium has enough biological capacity to keep ammonia and nitrite undetectable on a suitable aquarium test kit while its inhabitants are producing waste.
The Nitrogen Cycle - made simple
ammonia → nitrite → nitrate
Nitrification, carried out by beneficial microorganisms
nitrate → plant uptake → decay → ammonia
Ammonification — decomposers return organic nitrogen to the water as ammonia
Regular water changes remove some nitrate from the aquarium, so a tank is not a completely closed cycle.
A bit more detail
Ammonia enters the water directly from fish and is also produced as faeces, uneaten food and decaying plant material break down.
Nitrification is the biological process that converts ammonia first into nitrite, then into nitrate.
-
Ammonia oxidisers carry out the first stage, converting ammonia into nitrite. These include both bacteria and archaea.
-
Nitrite oxidisers carry out the second stage, converting nitrite into nitrate. In freshwater aquaria, these are commonly Nitrospira-related organisms.
-
Some Nitrospira, known as comammox Nitrospira, can carry out the complete conversion from ammonia to nitrate themselves.
These microorganisms need oxygen and grow mainly within biofilms on filter media and other wet surfaces such as substrate, décor and plants. They multiply relatively slowly and must build up in sufficient numbers to process the waste produced by the aquarium’s inhabitants.
Your filter does much more than catch visible bits of dirt. It has two different components doing different jobs. Mechanical media, the sponges and floss, physically strain debris out of the water. Biological media, usually ceramic rings, bio-balls or sintered blocks, provide a large surface area for the microorganisms that carry out nitrification. So a filter isn't optional; it's the engine room of a healthy tank.
When you clean the filter, rinse biological media gently in water taken from the aquarium rather than under untreated tap water. Chlorine or chloramine can damage the microorganisms you have spent weeks establishing.
What are we aiming for?
A properly cycled aquarium has enough biological capacity to keep ammonia and nitrite undetectable on a suitable aquarium test while livestock are producing waste.
Same Cycle, different waste loads
-
Betta: a small fish with a relatively modest waste load. In a correctly sized, well-filtered aquarium, the biological filter can readily keep pace once the cycle has been established and its capacity confirmed by testing
-
Channa: predatory fish whose size, feeding and waste output vary greatly by species. They need filtration and biological capacity matched to the particular fish and its eventual adult size.
-
Arowana: very large predators with a heavy waste load. They require powerful biological and mechanical filtration, generous water volume and a well-established system.
Bottom Line:
The biology is the same — what changes is the filtration capacity needed to handle the fish’s waste load. Every species needs a fully cycled tank before arrival; larger fish and heavier waste loads simply need greater filtration capacity and enough biological media to keep up.
How Long does cycling Take?
A brand-new fishless cycle usually takes several weeks, and sometimes longer. Every aquarium develops at a different rate, so the calendar is only a rough guide.
A common pattern is:
-
Ammonia begins to fall as ammonia oxidisers establish.
-
Nitrite then rises and later falls as nitrite oxidisers establish.
-
Nitrate often accumulates, although live plants may use some as it forms.
Cycling may be faster when you use:
-
mature filter media from a healthy, disease-free aquarium;
-
a suitable bottled microorganism product;
-
good oxygenation and plenty of filter-media surface area;
-
a stable temperature appropriate for the aquarium and the livestock it will eventually house
Our practical readiness check: Before adding livestock, the biofilter should repeatedly process the recommended test dose to 0 ppm ammonia and 0 ppm nitrite on your test kit within about 24 hours. Different aquariums mature at different rates, so consistency matters more than reaching a particular number of days. Do not redose while nitrite is still elevated.
Three Ways to Establish a Cycle
Start with fishless cycling. Mature filter media or a suitable bottled microorganism product can help the biofilter establish more quickly, but readiness still needs to be confirmed by testing.
Our recommended method for a new aquarium
-
Set up the aquarium with its filter, heater where required, substrate, plants and décor. Treat tap water for chlorine and chloramine, then run the filter continuously.
-
Add an aquarium-grade ammonium chloride or fishless-cycling product. This provides a controlled source of ammonia for the developing biofilter, effectively simulating the waste that livestock will eventually produce. Follow the product instructions for the correct dose.
-
Test ammonia and nitrite regularly. Redose according to the product instructions, but do not add another test dose while nitrite remains elevated.
-
Continue testing until the aquarium repeatedly processes the recommended test dose to 0 ppm ammonia and 0 ppm nitrite on your test kit within about 24 hours.
Method B — Add Mature Filter Media
The most effective way to accelerate cycling
Move established filter media or a mature sponge from a healthy, disease-free aquarium into the new filter. This introduces an existing community of microorganisms and can shorten the process significantly.
Continue supplying a measured ammonia source and confirm the aquarium’s capacity by testing. Mature media does not remove the need for the final 24-hour readiness check.
Method A — Classic Fishless Cycling
Method C — Add Bottled Micro-organisms
A useful additional boost
Reputable aquarium-cycling products add live microorganisms that may shorten the process. Follow the product’s storage, dosing and temperature instructions, then continue the fishless method and confirm readiness by testing.
Products differ, so never assume that adding a bottle has created an instant cycle.
Avoid: fish-in cycling: Exposing livestock to ammonia and nitrite while the biological filter develops is stressful and potentially harmful. Broadheath Bettas recommends preparing and confirming the cycle before livestock arrives.
Preparing Your Aquarium Before Livestock Arrives
Start several weeks before arrival:
-
Choose a correctly sized aquarium for the species.
-
Add the filter, heater where required, substrate, plants and décor.
-
Treat tap water with a conditioner that neutralises both chlorine and, where present, chloramine.
-
Run the filter continuously throughout the cycling process.
-
Add a measured ammonia source and follow the fishless-cycling method described above.
-
Allow the biological filter to establish. The microorganisms grow mainly within largely invisible biofilms on filter media and other wet surfaces; visible dirt or brown residue does not prove that the tank is cycled.
-
Keep testing until the aquarium can repeatedly process the recommended test dose to 0 ppm ammonia and 0 ppm nitrite on your test kit within about 24 hours.
Do not order livestock until the cycle has been confirmed by testing.
Testing and Monitoring
What you need
Use a reliable liquid test kit that measures ammonia, nitrite, nitrate and pH. The API Freshwater Master Test Kit is one widely used option, but other dependable kits are suitable.
Follow the instructions exactly, check the expiry date and record every result.
During the cycle
-
Before adding the ammonia source, test the aquarium water to establish its starting readings.
-
After adding ammonium chloride, allow it to circulate thoroughly, then test to confirm that you have reached the dose recommended by the product.
-
Test ammonia and nitrite daily during the cycle; check nitrate and pH regularly.
-
Do not add another test dose of ammonia while nitrite remains elevated.
-
For the final readiness check, test immediately before dosing and again approximately 24 hours later.
Before adding your livestock - ensure
-
Ammonia: 0 ppm
-
Nitrite: 0 ppm
-
Nitrate: preferably below ~20 ppm
-
pH: stable and appropriate for the species
Important to bear in mind
Nitrate often rises during cycling, but it may not visibly accumulate in a planted aquarium because plants can absorb nitrogen as it forms. The decisive test is therefore the repeatable 24-hour ammonia-and-nitrite readiness check, not the nitrate result alone.
Test daily during cycling and throughout the first week after livestock is added; once the aquarium is stable, test weekly and whenever behaviour or water quality gives cause for concern.
Keep a simple log: date, time, temperature, test results. This helps you spot problems early.
Settling In After Delivery
Your aquarium should already be fully cycled and ready before your new arrival is delivered.
Settling-in methods differ between species, so follow the instructions in the relevant care guide rather than using one generic acclimatisation routine.
Our Arrival and Your First Week guide explains what to check when the parcel arrives and the science behind our settling-in advice.
The exact settling-in method depends on the species:
Brief summary:
-
Most fish: keep the bag sealed and, if the bag and aquarium differ noticeably in temperature, float the sealed bag briefly, usually around 10–15 minutes. This adjusts temperature only. Then follow the prompt-transfer instructions in the relevant care guide.
-
Shrimp and dwarf crayfish: acclimatise gradually, unless the transport water is compromised.
-
Snails: briefly match the temperature, then transfer gently.
-
Axolotls: follow our separate quarantine-tub method.
For detailed settling-in advice, see the relevant species guide below.
Always discard the transport or acclimatisation water. Never add it to the aquarium.
First Week Monitoring
The first week after arrival is an important period. Even a properly cycled aquarium should be watched closely while the biological filter adjusts to its new waste load.
-
Test ammonia and nitrite daily for the first seven days. Both should remain at 0 ppm.
-
If either ammonia or nitrite is detected, carry out an immediate partial water change using dechlorinated, temperature-matched water. Retest afterwards and repeat as necessary until both return to zero.
-
Check the fish closely each day. Look for normal breathing, improving colour, appropriate activity and, once feeding begins, a healthy appetite.
-
Keep the temperature stable and ensure that the filter flow and oxygenation are suitable for the species.
-
Feed lightly at first and remove uneaten food promptly. Overfeeding adds unnecessary waste while the aquarium is adjusting.
-
Check the filter is running continuously and that its intake and outlet are unobstructed.
-
Record your test results so that any change can be recognised quickly.
For tropical fish:
For larger fish or heavy feeders, ammonia may rise more quickly, so closer testing and larger or more frequent water changes may be necessary.
For coldwater fish and other animals: follow the advice in their specific care guide.
Ammonia and nitrite should remain at 0 ppm on your test kit.
Never wait for a scheduled water-change day if either is detected.
Common Cycling Mistakes — and How to Avoid Them
-
Adding livestock before the cycle is confirmed
Ammonia and nitrite may rise to harmful levels. Add livestock only after the aquarium has repeatedly passed the 24-hour readiness check. -
Adding too many fish at once
A sudden increase in waste can exceed the filter’s established capacity. Increase stocking gradually where the aquarium is intended to house several fish. -
Cleaning or replacing too much filter media
Most of the biological community lives on the media. Rinse it gently in removed aquarium water and replace only part of it at a time when necessary. -
Using untreated tap water on filter media
Chlorine or chloramine may damage the microorganisms. Never rinse biological media directly under untreated tap water. -
Allowing the filter to stop for prolonged periods
The microorganisms need oxygenated water. Keep the filter running continuously and restore flow promptly after maintenance or a power interruption. -
Using soap or household cleaning products
Residues may harm livestock and disrupt the aquarium. Clean aquarium equipment with plain water or an aquarium-safe product only. -
Assuming the tank is cycled without testing
Clear water, brown residue or an elapsed number of weeks cannot confirm readiness. Readiness is confirmed by testing
Useful precaution
Keep a spare sponge or small bag of biological media running in a healthy established aquarium. It can provide mature media for a new setup or help after an emergency, but the receiving aquarium must still be tested before livestock is added.
What is really happening in a cycled aquarium
The Invisible Biology Keeping Your Fish Safe
Where ammonia comes from
Fish produce ammonia as they metabolise protein and amino acids. In most freshwater fish, the gills are the main route by which this nitrogenous waste leaves the body; urine makes a much smaller contribution.
Ammonia also enters the aquarium as faeces, uneaten food and dead plant material are broken down by microorganisms. This process is called ammonification and is separate from nitrification, which is carried out mainly by microorganisms living in the filter and on other wet surfaces.
Why the gills matter, and why our air-breathers are different
Bettas and channa can also take oxygen from atmospheric air using specialised structures above the gills: bettas have a labyrinth organ, while channa have suprabranchial air-breathing chambers. These are accessory respiratory organs—not gills adapted to breathe air.
Air-breathing fishes vary considerably in gill structure, physiology and how they handle nitrogenous waste. Some reduce ammonia production by suppressing amino-acid breakdown, convert nitrogen into less toxic compounds or temporarily tolerate raised internal ammonia.
Even closely related species differ: research on larval anabantoids found that Betta splendens had substantially larger, better-developed gills than the more air-dependent blue gourami.
These adaptations do not protect fish from poor water quality. Access to atmospheric air helps a fish obtain oxygen; it does not make ammonia or nitrite in the water safe. Both should be undetectable on a suitable aquarium test in a properly cycled aquarium.
Ammonia, ammonium and pH
Many common aquarium ammonia tests report total ammonia, which includes un-ionised ammonia, NH₃, and ammonium, NH₄⁺.
NH₃ is the more toxic form, and the proportion present as NH₃ rises with pH and temperature. This does not mean a low-pH aquarium can safely contain measurable total ammonia. In a stocked, established aquarium, an unexpected confirmed ammonia reading should be investigated rather than dismissed simply because the pH is low.
Should You Drip Acclimatise? Our species-specific approach
Ammonia also features in the debate over whether courier-delivered fish should be drip-acclimatised or moved promptly from their transport water.
Our approach is to minimise time spent in opened transport water and transfer most fish promptly after any necessary temperature matching.
Because the safest method depends on the species and the condition of the transport water, our Arrival and Your First Week guide explains the science behind our advice, while the relevant care guide gives the settling-in method for that species.
Scientific references and further reading
The following sources support the microbiology, fish physiology and water-chemistry information used in this guide. Brief notes explain why each reference is relevant.
Hovanec, T.A., Taylor, L.T., Blakis, A. and DeLong, E.F. (1998). Nitrospira-like bacteria associated with nitrite oxidation in freshwater aquaria. Applied and Environmental Microbiology, 64(1), 258–264. Showed that Nitrospira-related bacteria, rather than the traditionally cited Nitrobacter, were strongly associated with nitrite oxidation in freshwater aquarium biofilters. Sauder, L.A. et al. (2011). Aquarium nitrification revisited: Thaumarchaeota are the dominant ammonia oxidizers in freshwater aquarium biofilters. PLoS ONE, 6(8), e23281. Demonstrated that ammonia-oxidising archaea can play a major role in ammonia oxidation within freshwater aquarium biofilters. Bagchi, S. et al. (2014). Temporal and spatial stability of ammonia-oxidizing archaea and bacteria in aquarium biofilters. PLoS ONE, 9(12), e113515. Examined how ammonia-oxidising bacteria and archaea are distributed within aquarium biofilters and how stable these communities remain over time. Sauder, L.A. et al. (2018). “Candidatus Nitrosotenuis aquarius,” an ammonia-oxidizing archaeon from a freshwater aquarium biofilter. Described an ammonia-oxidising archaeon isolated from a freshwater aquarium biofilter, providing direct evidence of this group’s involvement in aquarium nitrification. McKnight, M.M. and Neufeld, J.D. (2024). Comammox Nitrospira among dominant ammonia oxidizers within aquarium biofilter microbial communities. Applied and Environmental Microbiology, 90(7), e00104-24. Found that complete ammonia-oxidising, or comammox, Nitrospira can be among the dominant ammonia oxidisers in freshwater aquarium biofilters. McKnight, M.M., Szabolcs, N., Graham, A. and Neufeld, J.D. (2025). Microbial community succession of home aquarium biofilters associated with early establishment of comammox Nitrospira. ISME Communications, 5(1), ycaf212. Followed three newly established home aquariums for 12 weeks and showed that nitrifier succession varied between tanks, with ammonia and nitrite becoming undetectable by week 3 in two aquariums and week 8 in the third. The study also found early establishment of comammox Nitrospira and reinforces why cycling should be confirmed by testing rather than by elapsed time alone.
Aquarium biofilters and nitrification
Fish ammonia excretion and air breathing
Randall, D.J. and Tsui, T.K.N. (2002). Ammonia toxicity in fish. Marine Pollution Bulletin, 45, 17–23. Reviews how fish produce and excrete ammonia, why the gills are the principal route of excretion in most species, and how environmental ammonia harms fish. Chew, S.F. and Ip, Y.K. (2014). Excretory nitrogen metabolism and defence against ammonia toxicity in air-breathing fishes. Journal of Fish Biology, 84(3), 603–638. Explains the varied ways air-breathing fishes manage nitrogenous waste and defend themselves against ammonia toxicity, while emphasising that the adaptations differ between species. Ip, Y.K., Chew, S.F. and Randall, D.J. (2004). Five tropical air-breathing fishes, six different strategies to defend against ammonia toxicity on land. Physiological and Biochemical Zoology, 77(5), 768–782. Shows that air-breathing fishes use several distinct physiological strategies during air exposure, including species-specific responses studied in Channa. Tate, M. et al. (2017). Life in a bubble: the role of the labyrinth organ in determining territory, mating and aggressive behaviours in anabantoids. Journal of Fish Biology. Provides useful background on the anatomy and wider biological role of the labyrinth organ in bettas and other anabantoid fishes. Mendez-Sanchez, J.F. and Burggren, W.W. (2019). Hypoxia-induced developmental plasticity of larval growth, gill and labyrinth organ morphometrics in two anabantoid fish. Journal of Morphology, 280, 193–204. Compared gill and labyrinth-organ development in Betta splendens and blue gourami, showing that closely related air-breathing fishes can differ substantially in their reliance on gills and accessory respiratory organs.
Ammonia chemistry
Emerson, K., Russo, R.C., Lund, R.E. and Thurston, R.V. (1975). Aqueous ammonia equilibrium calculations: effect of pH and temperature. Journal of the Fisheries Research Board of Canada, 32, 2379–2383. Established how pH and temperature alter the balance between un-ionised ammonia, NH₃, and ammonium, NH₄⁺, helping explain why ammonia toxicity rises in warmer, more alkaline water.