How long does it take to cycle a new aquarium?

Usually three to six weeks with the fishless method, but the time varies and is not fully controllable. It depends on temperature, the amount of ammonia supplied and above all how many bacteria you start with. Adding filter media or gravel from a healthy, established aquarium can shorten the wait noticeably, because you bring live bacteria straight in. Do not trust fixed deadlines: the cycle is done when the tests confirm it, not when a certain number of days has passed.

Can I speed up the cycle with bottled bacteria?

Bacteria products can help, but with variable results depending on the brand, storage and freshness. None of them makes the tank safe instantly, and anyone promising a complete cycle in a few hours should be taken with caution. The most reliable way to speed things up is still transferring mature filter media from a healthy tank. In any case, whatever method you use, the final proof is always the same: ammonia and nitrite at zero measured with a test, not a promise on the packaging.

How do I know the cycle is complete and I can add fish?

The tank is ready when it passes the double zero reading. In practice you add ammonia to a moderate level and measure it the next day: if within about twenty four hours both ammonia and nitrite are back to zero and only nitrate remains, the bacteria work fast enough to support fish. At that point do a generous water change to lower the nitrate built up during the cycle, then you can add the first fish, a few at a time.

I already added fish without cycling: what do I do now?

Switch to managing a cycle with the fish already in, aiming to keep ammonia and nitrite as low as possible while the bacteria grow. Measure daily and do partial water changes whenever ammonia or nitrite rises, to dilute them. Cut back on food, because less waste means less ammonia, and do not add more fish until things are stable. It is a demanding, stressful phase for the fish, but with regular water changes and patience many get through the critical period.

Non conventional species40 minLevel: BeginnerUpdated on August 20, 2026

Starting and cycling a freshwater aquarium before fish

How to complete the nitrogen cycle in a new aquarium: why it matters, the step by step method, the values to monitor and the mistakes that kill fish.

Audience: Pet owners
Species: Fish
Scope: Valid everywhere, Italy, European Union

What you need before you start

Un acquario già montato con filtro e riscaldatore, riempito e in funzione
Un test kit a reagenti liquidi per ammoniaca, nitriti e nitrati
Un biocondizionatore per eliminare il cloro dall'acqua di rubinetto
Una fonte di ammoniaca controllata, come mangime o ammoniaca pura senza additivi, per nutrire i batteri

The mistake that kills the most aquarium fish is not some exotic disease: it is adding them to a new tank on the very day it was filled. A freshly set up aquarium is clean but biologically dead water, without the bacteria that turn the fish's toxic waste into less dangerous substances. Adding fish before those bacteria have established means poisoning them with their own ammonia, and it is the cause of what is called new tank syndrome. This guide explains what the nitrogen cycle is, why it must be completed before any fish, how to run it step by step, which values to measure and which mistakes turn it into a predictable die off. The thresholds given are general references: the species you want to keep has its own needs and should be researched separately.

Why a new aquarium is not ready

Fish continuously produce nitrogen waste, mainly ammonia, which is toxic even at low concentrations. In a mature aquarium this ammonia is broken down by bacterial colonies living in the filter and on the tank surfaces: one group turns it into nitrite, equally toxic, and a second group turns nitrite into nitrate, far less dangerous and removed with water changes. This chain is the nitrogen cycle, and in a tank just filled it simply does not exist yet.

Ammonia: produced by fish and leftovers, toxic from the start.
Nitrite: the first product of bacterial conversion, still very toxic.
Nitrate: the end product, low toxicity at moderate levels and managed with water changes.
Bacteria: the colonies performing the two conversions, which must grow before the fish.

Cycling with or without fish

There are two ways to grow the bacteria. In fishless cycling you add a controlled ammonia source to an empty tank, feed the colony and wait for it to develop on its own, without risking any animal. In cycling with fish you use a few very hardy fish as ammonia producers, but expose them to weeks of harsh conditions. The responsible choice, now recommended, is fishless cycling.

Comparison of the two methods. Fishless cycling is preferable for animal welfare.
Aspect	Fishless	With fish
Welfare	No animal exposed to toxic levels	Fish under ammonia and nitrite for weeks
Control	Ammonia doses managed by you	Variable, hard to regulate output
Speed	Can be sped up with mature filter media	Bound by the few fish present
Risk	Low, at worst you lose time	High, with possible losses and chronic harm

Running the fishless cycle step by step

The fishless cycle takes patience but very little daily work: it is a matter of feeding the bacteria and measuring until they do their job on their own. From start to finish it can take three to six weeks, sometimes more, and there is no reliable way to force it beyond a certain point.

Set up and stabilise the tank
Fill with water treated with a conditioner to remove chlorine, start the filter and heater, and bring the temperature to warm values, which favour bacterial growth.
Introduce the ammonia source
Add a small amount of fish food or pure additive free ammonia to raise ammonia to a moderate level. It is the food that feeds the first bacterial colony.
Measure daily and take notes
Test ammonia, nitrite and nitrate and record the values. Watching the trend over time is the only way to know how far the cycle has progressed.
Wait for nitrite to appear
After some days the ammonia starts to fall and nitrite appears: proof that the first group of bacteria is working. Keep topping up ammonia without overdoing it.
Wait for nitrate to appear
When nitrite also drops and nitrate rises, the second group of bacteria is active. The cycle is almost complete.
Confirm the double zero reading
The tank is ready when, after adding ammonia, both ammonia and nitrite return to zero within a day and only nitrate remains. Before adding fish, do a generous water change to lower the accumulated nitrate.

Reference values and monitoring

During and after the cycle, the three nitrogen values tell the tank's health better than any visual impression. A liquid reagent test is more reliable than strips, which tend to be imprecise exactly at the low thresholds that matter. The figures below are general references for community freshwater: the species you keep may require different values.

General reference values for a stocked freshwater aquarium. Research the needs of the species you intend to keep.
Parameter	Target with fish	Note
Ammonia	0	Any detectable value is a risk to fish
Nitrite	0	Even traces are toxic, keep it at zero
Nitrate	Low and stable	Controlled with regular water changes
Temperature	Stable per species	Swings stress more than a slightly different value

Common mistakes that ruin the cycle

Almost every cycle failure comes from haste or apparently harmless acts that destroy the newly formed bacteria. Knowing these mistakes in advance is worth more than any miracle product, because most have no fix except starting the wait again.

Adding fish before ammonia and nitrite are steadily at zero, hoping it works out.
Washing the filter under chlorinated tap water, which kills the bacterial colony.
Changing all the water and cleaning everything at once, wiping out the bacteria across the tank.
Trusting test strips instead of liquid reagents, which are more precise at low thresholds.
Adding too many fish at once right after cycling, exceeding the bacteria's capacity.
Overfeeding the fish, because uneaten food becomes more ammonia to process.

Frequently asked questions

How long does it take to cycle a new aquarium?: Usually three to six weeks with the fishless method, but the time varies and is not fully controllable. It depends on temperature, the amount of ammonia supplied and above all how many bacteria you start with. Adding filter media or gravel from a healthy, established aquarium can shorten the wait noticeably, because you bring live bacteria straight in. Do not trust fixed deadlines: the cycle is done when the tests confirm it, not when a certain number of days has passed.
Can I speed up the cycle with bottled bacteria?: Bacteria products can help, but with variable results depending on the brand, storage and freshness. None of them makes the tank safe instantly, and anyone promising a complete cycle in a few hours should be taken with caution. The most reliable way to speed things up is still transferring mature filter media from a healthy tank. In any case, whatever method you use, the final proof is always the same: ammonia and nitrite at zero measured with a test, not a promise on the packaging.
How do I know the cycle is complete and I can add fish?: The tank is ready when it passes the double zero reading. In practice you add ammonia to a moderate level and measure it the next day: if within about twenty four hours both ammonia and nitrite are back to zero and only nitrate remains, the bacteria work fast enough to support fish. At that point do a generous water change to lower the nitrate built up during the cycle, then you can add the first fish, a few at a time.
I already added fish without cycling: what do I do now?: Switch to managing a cycle with the fish already in, aiming to keep ammonia and nitrite as low as possible while the bacteria grow. Measure daily and do partial water changes whenever ammonia or nitrite rises, to dilute them. Cut back on food, because less waste means less ammonia, and do not add more fish until things are stable. It is a demanding, stressful phase for the fish, but with regular water changes and patience many get through the critical period.

What to do next

Do not add fish to a new tank: first grow the bacteria that process ammonia and nitrite. Choose the fishless cycle, feed the colony with a controlled ammonia source and measure daily until, after an addition, ammonia and nitrite return to zero within a day leaving only nitrate. Do a water change, then add a few fish at a time while continuing to monitor the values.