Knowledge Centre · Starting a new aquarium
Starting a new aquarium: how to cycle it safely, step by step
A new aquarium needs its biological filter established before it can safely support livestock — this is called cycling, and it's the single most important thing to get right when starting a tank. The safest approach is fishless cycling: dosing an ammonia source into an empty, filtered, heated tank and testing regularly until ammonia and nitrite both convert to nitrate reliably, before any animal goes in. It commonly takes several weeks, sometimes longer, and there's no fixed calendar date — testing is what actually confirms progress. Once your tank is genuinely cycled, adding livestock gradually over several weeks, rather than fully stocking on day one, gives your bacterial population time to keep pace with the extra waste. The sections below walk through what to prepare, the process itself, a simple testing schedule, and how to add your first fish. Still weighing this up against saltwater or a planted setup? Our freshwater vs saltwater comparison is worth reading before you commit to equipment either way.
At a glance
What cycling actually is
Establishing the bacteria that convert fish waste (ammonia) into nitrite and then the far less harmful nitrate, before livestock depend on that process.
The safest way to start
Fishless cycling — dosing ammonia into an empty tank and testing until it's reliably converted — avoids exposing any animal to ammonia or nitrite at all.
Sensible immediate response
If fish are already in an uncycled tank, test ammonia and nitrite daily and be ready to do a prompt partial water change rather than waiting for symptoms to appear.
Why a new tank needs to cycle
Fish waste, uneaten food and other organic matter break down into ammonia, which is harmful to aquatic life — in a stocked tank, any detectable reading is worth investigating and acting on promptly. In an established aquarium, colonies of bacteria living on the filter media, substrate and decor convert that ammonia into nitrite and then into the far less harmful nitrate — the aquarium nitrogen cycle. A brand-new tank doesn't have those bacterial colonies yet, so there's nothing to process ammonia until they've had time to establish. Cycling is simply the process of growing that colony to a working size before it's needed.
This guide focuses on the practical steps of starting a tank from scratch. For the underlying science — the three-stage process, the specific microorganisms involved, and why clear water doesn't mean a tank is cycled — see our full guide to the aquarium nitrogen cycle. Once you've followed the steps below and are wondering whether your specific tank is actually ready for livestock, see our decision guide to whether a tank is genuinely cycled for a practical framework rather than a guess.
What to set up before you start
Cycling happens inside a tank that's already running as it would with livestock, minus the livestock itself. Have the following in place and operating before you begin:
Not sure of your tank's volume yet? Use the free Aquarium Calculator to work out litres or gallons from its dimensions, plus its approximate water weight — useful for checking a stand or floor can take the load before you fill it.
- The tank itself, filled with dechlorinated water — treated with a conditioner appropriate to your water supplier's treatment (see the cycling steps below for why the specific conditioner matters).
- A running filter, sized appropriately for your tank, since this is where most of the bacterial colony will actually live. The free Aquarium Filter Turnover Calculator can help you check its flow rate against your tank's volume.
- A heater and thermometer, if you're keeping tropical species, set to your intended stocking temperature — nitrifying bacteria are temperature-sensitive, so cycling at the temperature you'll actually run reduces surprises later. If you haven't chosen a heater yet, the free Aquarium Heater Calculator gives a sensible wattage range to shop from.
- Substrate and decor already in place, since these also provide surface area for bacteria to colonise.
- A liquid test kit that measures ammonia, nitrite and nitrate — test strips are less precise for the low concentrations that matter most during cycling.
- An ammonia source for fishless cycling — a product intended for this purpose, dosed and tracked according to its own instructions.
Filling the filter with mature media from an already-established, healthy tank, where available, can meaningfully shorten cycling by seeding it with an existing bacterial population[4]. This isn't available to everyone starting a first tank, but it's worth asking a fellow fishkeeper if you know one.
Choosing a cycling method
Fishless cycling means establishing the bacterial colony before any animal is added, typically by dosing the tank with an ammonia source and tracking how it's processed through to nitrate over time. Because no livestock are present, there's no welfare risk while ammonia and nitrite rise and fall as part of the normal process. This is the approach the rest of this guide walks through, and it's the widely preferred starting point for a genuinely new tank[2].
Cycling with livestock present — sometimes called a fish-in cycle — means the bacterial population establishes while animals are already in the tank, so any ammonia or nitrite produced is a direct welfare concern rather than a reading to observe from a distance. It's sometimes unavoidable, for example if fish were already added before cycling was understood, but it isn't the recommended way to start a tank from scratch. If you're in this situation now, see the FAQ below and our ammonia and nitrite guides for the specific immediate steps.
Fishless cycling, step by step
Set up
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Fill the tank and start the filter
Add substrate and decor, fill with water, and get the filter and heater running continuously from this point on — the bacteria you're growing need constant flow and oxygen to establish.
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Condition the water correctly
Use a conditioner appropriate to your water supplier's treatment. Many suppliers use chloramine, which combines chlorine and ammonia, and a conditioner that only addresses chlorine can leave the ammonia portion behind[1][5]. Follow the product's own instructions exactly, since formulations vary.
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Test your source water once, before dosing anything
Some tap water already carries a trace of ammonia or nitrate. Knowing your true starting point means you won't misread it as progress (or a problem) later.
Establish ammonia-to-nitrite
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Dose an ammonia source
Add an ammonia source intended for fishless cycling, following that product's own dosing instructions rather than a figure from an unrelated method. Record what you added and when.
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Test ammonia and nitrite every few days
Watch for ammonia beginning to fall between doses as the first bacterial population establishes, and don't be concerned if nitrite starts appearing and rising at this stage — that's expected, not a setback.
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Keep re-dosing as your method instructs
Most fishless methods have you continue adding ammonia on a schedule (or once existing ammonia clears) to keep feeding the developing bacterial colony until it's consistently converting a full dose.
Establish nitrite-to-nitrate
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Expect nitrite to rise before it falls
A second, slower-establishing bacterial population converts nitrite into nitrate. Ammonia-oxidising and nitrite-oxidising microorganisms typically establish at different rates, so a nitrite spike after ammonia has started clearing is a normal and expected part of the process, not a sign anything has gone wrong[4].
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Keep testing through the nitrite stage
Continue testing ammonia and nitrite regularly and watch for nitrate appearing — the presence of nitrate is your evidence that the full sequence is working, not just the first stage.
Confirm completion
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Dose a full amount and test 24 hours later
A tank is typically considered close to fully cycled once a full dose of ammonia reads back at zero for both ammonia and nitrite within about 24 hours, with nitrate present. Check your specific method's own completion criteria, since exact confirmation steps vary between methods.
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Do a partial water change before adding any livestock
This brings accumulated nitrate down to a sensible starting level and gives you a clean baseline reading before the tank has to support animals.
A simple testing schedule while you wait
There's no fixed calendar date when a tank finishes cycling — testing is what tells you where you actually are. The stages below describe the pattern to expect, not a promised timeline.
| Stage | What you're likely to see | What to do |
|---|---|---|
| Early | Ammonia present and holding steady or slowly rising after dosing; nitrite and nitrate close to zero | Test every 2–3 days; keep dosing as your method instructs |
| Ammonia converting | Ammonia starting to fall between doses; nitrite beginning to appear | Keep testing both; don't be concerned by the nitrite reading |
| Nitrite peak | Ammonia consistently at or near zero; nitrite at its highest | Test every 2–3 days; nitrate should start appearing soon |
| Nitrite converting | Nitrite starting to fall; nitrate rising and clearly present | Continue testing; you're approaching completion |
| Cycled | A full ammonia dose reads zero for ammonia and nitrite within ~24 hours, with nitrate present | Confirm against your method's completion criteria, do a water change, then add livestock gradually |
Adding your first fish
Once your tank is genuinely cycled, resist the temptation to stock it fully on day one. Adding livestock gradually — a small number at a time, spaced over several weeks — gives your bacterial population, which has only ever processed a dosed ammonia source or a smaller stocking level, time to keep pace with the extra waste each new addition brings[2]. A sudden jump in bioload is one of the most common ways a newly cycled tank ends up with a detectable ammonia or nitrite reading shortly after fish arrive.
- Add a small first group, then wait one to two weeks, testing ammonia and nitrite throughout, before adding more.
- Acclimate new arrivals properly — matching temperature (and salinity, for marine tanks) before release reduces stress on top of everything else the tank is adjusting to.
- Test after every addition, not just during the original cycling process — a bioload increase can produce a small, temporary rise even in an otherwise stable tank.
- Hold off on more livestock if a reading appears, and treat it the same way you would in any stocked tank — see our ammonia and nitrite guides.
- Use the free Water Change Assistant if you need to work out how much water to change to bring a reading down after a new addition.
Common new-tank mistakes
- Adding fish on the same day the tank is set up, before any bacterial colony has had a chance to establish.
- Assuming clear, odourless water means a tank is cycled — clarity says nothing about ammonia or nitrite; only testing does.
- Fully stocking a newly cycled tank in one go rather than adding livestock gradually.
- Rinsing new filter media, substrate or decor in tap water once bacteria have started to establish, rather than in removed tank water.
- Switching the filter off overnight or between water changes — the bacteria you're growing need continuous flow and oxygen.
- Relying on a bottled bacteria product alone without testing to confirm the tank is actually processing waste.
- Giving up on testing once things "seem fine" — a newly cycled tank's bacterial population is still relatively small and can be disrupted more easily than a long-established one.
Signs of trouble and when to get help
If livestock are already present while your tank is still establishing, watch for gasping at the surface, rapid or laboured breathing, unusual hiding, loss of balance, or appetite loss — all of which can indicate ammonia or nitrite exposure. Test immediately if you see any of these, and don't wait for a scheduled test.
If you suspect an already-cycled tank has lost its bacterial colony — for example after a power cut, an overly aggressive filter clean, or certain medications — see our dedicated guide to recognising and responding to a possible cycle crash.
Setting yourself up for the long term
- Keep a simple written record of your dosing and test results while cycling — it's easy to lose track of the pattern from memory alone.
- Decide your intended stocking and temperature before you start, and cycle at that temperature, so you're not adjusting several things at once later.
- Once livestock are added, settle into a regular water-change routine rather than reacting only when a reading looks wrong[3].
- Keep testing periodically even once the tank feels established — a stable environment is something you maintain by checking it, not something you set up once and stop monitoring.
Frequently asked questions
How long does it take to cycle a new aquarium?
There's no fixed number of days or weeks that applies to every tank — it commonly takes several weeks, and sometimes longer, depending on your method, water temperature, available surface area for bacteria, and whether the tank was seeded with mature filter media. Follow your chosen method's own completion criteria and confirm with testing rather than a calendar date.
Do I need fish in the tank while it cycles?
No, and it's generally better if you don't. Fishless cycling lets ammonia and nitrite rise and fall as expected without exposing any animal to them, which is why it's the widely preferred approach for a brand-new tank. Cycling with livestock already present is possible but turns every ammonia or nitrite reading into a direct welfare concern rather than an abstract number.
Can I add all my fish at once once the tank is cycled?
Adding livestock gradually over several weeks is safer than stocking fully on day one, even in a genuinely cycled tank. A sudden jump in bioload can outpace what your bacterial population has actually been tested against, since it has only been processing whatever ammonia source or smaller stocking level you used during cycling.
Do bottled bacteria products let me skip cycling?
Some products can help seed a tank with nitrifying bacteria and may shorten the process, but effectiveness varies by product and by what's already establishing naturally in your system. Testing ammonia and nitrite directly is still the only reliable way to confirm your tank is actually processing waste, regardless of which products you've used.
What if I've already added fish before reading this?
You're now cycling with livestock present, which is manageable but needs closer attention: test ammonia and nitrite daily for now, and be ready to do a prompt partial water change with conditioned, temperature-matched water if either is detected. See our ammonia and nitrite guides for the specific steps, and hold off adding further fish until the tank has finished cycling.
Is a planted or saltwater tank cycled differently?
The underlying biology is the same nitrogen cycle in every case. Live plants can take up some ammonia and nitrate directly once established, which may ease things slightly, but they don't replace the need to cycle a new tank. Saltwater and reef systems follow the same fishless-cycling logic, with salinity matched alongside temperature and, for reef tanks, often a longer settling period before adding sensitive invertebrates.
If something looks wrong while you're cycling
The theory in this guide is the easy part. Living through the several weeks it describes, with your own readings moving in ways that don't always match the timeline you expected, is what actually tests you. If a reading looks off right now, the free Emergency Diagnosis tool can help you prioritise what to check first — it doesn't replace the cycling process this guide describes, just helps with a specific moment in it.
Watching your own numbers move, not just reading about it
Nothing here replaces actually understanding the nitrogen cycle — that's what the rest of this guide is for. Once you're testing regularly, though, AquaPulse's Water Check can hold each result alongside the last, so you're watching your own tank's numbers move day by day instead of relying on memory or a spreadsheet you have to open separately.
Water Check works from the readings you enter — it doesn't take automatic readings and isn't a substitute for testing your water with a reliable kit, or for veterinary advice if livestock are seriously or persistently unwell. For the plain-language explainer without the app, see the Water Check feature page.
Sources and editorial information
- Published
- 25 July 2026
- Last reviewed
- 25 July 2026
- Written and maintained by
- AquaPulse (Valeon Labs Ltd)
This guide is written and maintained by the AquaPulse team at Valeon Labs Ltd. It draws on published aquatic-science, veterinary and animal-welfare references rather than a single author's personal experience, and we paraphrase and link to those sources rather than reproducing their wording. We'll update this page if the sources it's based on change, or if we find something we've got wrong.
Good to know: This guide provides general informational guidance only. It is not a veterinary service and does not replace specialist or aquatic health advice for serious livestock health issues.
- [1] University of Florida IFAS Extension — "Ammonia in Aquatic Systems" (FA16/FA031) Used for background on chloramine treatment of tap water and general ammonia toxicity in aquatic systems.
- [2] RSPCA — "Choosing an Aquarium for Pet Fish" Used for the welfare case for fishless cycling before adding livestock, and for adding fish gradually rather than fully stocking a new tank at once.
- [3] Merck Veterinary Manual — "Management of Aquarium Fish" Used for general new-system management guidance consistent with our other water-quality guides, including gradual stocking and consistent maintenance routines.
- [4] Hovanec & DeLong (1996), "Comparative Analysis of Nitrifying Bacteria Associated with Freshwater and Marine Aquaria," Applied and Environmental Microbiology Used for how ammonia-oxidising and nitrite-oxidising bacterial populations establish at different rates, and for seeding a new filter with mature media.
- [5] US Environmental Protection Agency — "Basic Information about Chloramines and Drinking Water Disinfection" Used for how chloramine treatment of tap water can introduce ammonia if a conditioner isn't specifically labelled for it.