hobby calculator

Fish Tank Cycling Time Calculator

Estimate aquarium cycling time from temperature, pH, seeding, ammonia level and method, with a phase breakdown.

A fishless cycle at 25 C and pH 7.6 with no seeding takes about 28 days, inside the published three-to-eight-week range. Nitrifiers roughly double their rate per 10 C, pH below 6.5 can stall the cycle, and established filter media cuts it by about 60%. A warm seeded planted tank finishes in about 7 days; a cool acidic over-dosed fish-in cycle takes around 90. The ammonia phase is about 40% of the total and the slower nitrite phase the rest.

Quick actions: Save inputs, view saved runs, print, share, or export results

Saved Runs

Inputs

Method

Seeded with

Tank Temperature°C

Ammonia Targetppm

Tank VolumeL

Filter Turnover× per hour

Live plants

Results

Estimated Cycle Time

28 days

In Weeks4.0 weeks

Realistic Range20 to 41 days

Ammonia Phase11.2 days

Nitrite Phase16.8 days

Temperature Factor1.00

pH Factor1.00

Seeding Factor1.00

Ammonia Level Factor1.00

Method Factor1.00

Live Plants Factor1.00

Filter Turnover Factor1.00

Ammonia Dose for This Tank20.0 mL

Test Every2 days

Days Saved by Warming to 30 °C8.2 days

Tip

Worth checking before adding fish: the difference between a warm seeded tank and a cool unseeded one is a week against three months.

Formula

Cycling time model

Estimated days = 28 x temperature x pH x seeding x ammonia x method x plants x turnover
  (floored at 7 days)

Temperature  2^((25 - T) / 10), with penalties below 15 C and above 30 C
pH           1.00 above 7.4 | 1.15 at 7.0-7.4 | 1.50 at 6.5-7.0 | 2.20 below 6.5
Seeding      1.00 none | 0.65 bottled | 0.55 used gravel | 0.40 established media
Ammonia      1.00 at 1-3 ppm | 1.08 at 3-4 | 1.25 at 4-5 | 1.45 above 5 | 1.20 below 1
Method       1.00 fishless | 1.30 fish-in
Plants       0.88 with live plants

Ammonia phase = 40% of the total, nitrite phase = 60%

The 28-day reference is a fishless, unseeded cycle at 25 °C and pH 7.6 dosed to 2 ppm, which sits in the middle of the published three-to-eight-week range. Each factor multiplies it. The temperature term uses a Q10 of 2, the standard doubling per ten degrees for nitrifying bacteria; the pH terms reflect that these bacteria need carbonate and work poorly in acidic water; and the ammonia term captures the fact that above about 4 ppm the nitrite oxidisers are inhibited, so over-dosing slows the cycle rather than speeding it.

Step by step

How the calculation works

1The estimate places your tank inside the published three-to-eight-week range rather than repeating the range. The optimistic and pessimistic bounds are there because biology varies enough between identical setups that a single number would mislead.
2The two phases behave differently. Ammonia oxidisers establish first and account for about 40% of the wait; nitrite oxidisers are slower and account for the rest. A tank that clears ammonia quickly and then sits at high nitrite for weeks is normal, not stalled.
3Raising the temperature is the cheapest acceleration available — going from 25 to 30 °C saves around eight days on a four-week cycle. Dropping pH below 7.0 is the most common thing that stalls one.
4Dose to about 2 ppm and no higher. Above 4 ppm the nitrite-oxidising bacteria are actively inhibited, so over-dosing makes the cycle slower. Do not change water during a fishless cycle unless nitrite goes off the chart.
5The test that matters: dose to 2 ppm, wait 24 hours, and if both ammonia and nitrite read zero with nitrate present, the tank is cycled. Your test kit is the authority, not this estimate. The [aquarium bioload calculator](/calculators/aquarium-bioload-stocking-calculator) decides what the new filter can support.

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Aquarium Volume Guide: Gallons, Liters, and Usable Water Capacity

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Hobby Tool

Aquarium Bioload & Stocking Calculator

What the new filter can actually support once the cycle finishes.

Hobby Tool

Aquarium Volume Calculator

The real water volume after substrate and hardscape, which every dose is calculated against.

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Aquarium CO2 Injection Calculator

The next question a planted tank raises, using the same carbonate chemistry.

hobby calculator

Fish Tank Cycling Time Calculator

Estimate aquarium cycling time from temperature, pH, seeding, ammonia level and method, with a phase breakdown.

Quick actions: Save inputs, view saved runs, print, share, or export results

Saved Runs

Inputs

Method

Seeded with

Tank Temperature°C

Ammonia Targetppm

Tank VolumeL

Filter Turnover× per hour

Live plants

Results

Estimated Cycle Time

28 days

In Weeks4.0 weeks

Realistic Range20 to 41 days

Ammonia Phase11.2 days

Nitrite Phase16.8 days

Temperature Factor1.00

pH Factor1.00

Seeding Factor1.00

Ammonia Level Factor1.00

Method Factor1.00

Live Plants Factor1.00

Filter Turnover Factor1.00

Ammonia Dose for This Tank20.0 mL

Test Every2 days

Days Saved by Warming to 30 °C8.2 days

Tip

Worth checking before adding fish: the difference between a warm seeded tank and a cool unseeded one is a week against three months.

Summary

Key output metrics

Estimated Cycle Time

28 days

About 4.0 weeks, realistically 20 to 41 days

Ammonia Phase

11 days

Then 17 days for the slower nitrite stage

Ammonia Dose

20 mL

To bring 100 L to 2 ppm — test every 2 days

D3 visualization

What Each Factor Multiplies the Cycle By

Values above 1 lengthen the cycle and values below shorten it. Everything is applied to a 28-day reference case: fishless, unseeded, 25 °C, pH 7.6, dosed to 2 ppm.

Guide

How to understand and use this calculator

Quick Answer — How Long Does It Take to Cycle a Tank?

The published range for a fishless cycle is three to eight weeks, which is true and unhelpful. What decides where a specific tank lands is bacterial growth, so the drivers are the ones that govern bacteria: temperature, pH, whether you seeded the filter, and how much ammonia the colony is being asked to process.

The reference case — a fishless cycle at 25 °C, pH 7.6, no seeding, dosed to 2 ppm ammonia — takes about 28 days. From there:

Temperature — nitrifiers roughly double their rate for every 10 °C, so 30 °C is markedly faster than 20 °C, and below 15 °C the cycle crawls
pH — below 7.0 the process slows noticeably, and below 6.5 it can stall entirely because nitrifying bacteria need carbonate
Seeding — bottled bacteria cut the time by about a third, used gravel by nearly half, and media moved from an established filter by about 60%
Ammonia level — around 2 ppm is ideal; above 4 ppm the nitrite-oxidising bacteria are inhibited and the second half stalls

A warm, seeded, planted tank can finish in about a week. A cool, acidic, over-dosed fish-in cycle can take three months. Both are the same process.

How to Use This Calculator: A Standard Fishless Cycle

Enter your tank temperature, pH, what you seeded the filter with, the ammonia level you are dosing to, and your filter turnover rate. The calculator applies each factor to the 28-day reference case and reports the phases separately.

For a 100-litre tank at 25 °C and pH 7.6, unseeded, dosed to 2 ppm with a filter turning over 5 times an hour, every factor sits at 1.0 and the estimate is the reference 28 days — about four weeks. The realistic range runs from 19.6 to 40.6 days, which is the honest spread for a biological process.

The phases matter because they behave differently. The ammonia-processing phase takes about 11.2 days and the nitrite phase about 16.8. Ammonia oxidisers establish first and faster; nitrite oxidisers are slower-growing and account for the back half of the wait. That is why a tank that clears ammonia in a fortnight and then seems to sit at high nitrite for weeks is behaving exactly as expected rather than stalling.

The calculator also reports the ammonia dose: about 20 mL of a standard aquarium ammonia solution to bring 100 litres to 2 ppm. Dose, test daily, and top back up to 2 ppm as it falls. And it suggests testing every two days for a fishless cycle — daily is unnecessary and wastes reagent.

One useful counterfactual: raising this tank to 30 °C would save about 8.2 days. A heater turned up for a month is the cheapest acceleration available. Once the cycle is complete, the aquarium bioload and stocking calculator decides how much livestock the new filter can actually support.

A Second Example: The Fastest and Slowest Cases

A well-prepared tank: 200 litres at 29 °C, pH 7.8, seeded with media from an established filter, 2 ppm ammonia, live plants, 6× turnover. Every factor pushes the same way — temperature 0.76, seeding 0.40, plants 0.88, turnover 0.95 — and the estimate falls to 7.1 days, with a range of 5.0 to 10.3. The ammonia phase is under three days and the nitrite phase just over four.

That is about as fast as a cycle can honestly be described. The calculator will not report less than seven days regardless of the inputs, because even a filter moved wholesale from a mature tank has to expand its colony to the new bioload, and no published account of a completed cycle runs shorter.

The opposite case: 60 litres at 22 °C, pH 6.6, no seeding, dosed to 5 ppm, fish-in, 3× turnover. Now every factor works against you — temperature 1.23, pH 1.50, ammonia 1.25, method 1.30, turnover 1.08 — and the estimate is 90.7 days, roughly 13 weeks, with a range from 63.5 to 131.6 days.

Three of those penalties are avoidable. The pH at 6.6 is the largest single problem and can be corrected with a small carbonate addition. The 5 ppm ammonia dose is actively counterproductive and should be 2. And a fish-in cycle is slower by design because water changes to protect the fish keep removing the ammonia the bacteria need. Fixing all three would cut this to a few weeks. Testing daily is correct here, because fish are in the tank and their exposure to ammonia is the constraint. For the water volume the doses are calculated against, the aquarium volume calculator gives the real figure rather than the nominal tank size.

How to Tell When the Cycle Is Actually Done

One test, and it is unambiguous. Dose to 2 ppm ammonia. Twenty-four hours later, both ammonia and nitrite read zero, and nitrate is present. If that happens, the colony can process a full day's bioload overnight and the tank is cycled. If ammonia has fallen but nitrite is still detectable, the second stage is not finished yet — keep going.

Nitrate appearing is the confirmation, not a side effect. It is the end product of the two-stage conversion, and its presence proves both bacterial populations are working. A tank with zero ammonia, zero nitrite and zero nitrate is not cycled; it is a tank where nothing has happened yet.

Some practical points that shorten a cycle or prevent it stalling:

Do not change water during a fishless cycle unless nitrite goes off the chart. Water changes remove the ammonia and nitrite the bacteria are growing on, which is why fish-in cycles take longer.

Do not use dechlorinated water as an excuse to skip dechlorination. Chlorine and chloramine kill nitrifying bacteria, so any top-up water needs treating even mid-cycle.

The bacteria live in the filter, not the water. Roughly 90% of the colony is on filter media and surfaces. Replacing a filter cartridge the week after cycling completes removes most of what you just spent a month growing — clean media in old tank water instead, and never under a tap.

Live plants genuinely help, absorbing ammonia directly and reducing the load on the bacteria, which is why the calculator applies a modest bonus for them. Once the tank is planted and running, the aquarium CO2 injection calculator covers the next question a planted tank raises.

Limitations

This is an estimator built on documented relationships, not a measurement. The base case, the temperature response, the pH thresholds and the seeding factors are drawn from published nitrification behaviour and from the range of cycling times aquarists consistently report — but the underlying biology varies enormously between tanks for reasons no calculator can see. Two identical setups can differ by a week. The optimistic and pessimistic bounds are there because a single number would be misleading.

The seeding factors in particular are approximations of something highly variable. "Media from an established filter" covers everything from a squeezed sponge to half a mature filter's contents, and the difference between those is substantial. Bottled bacteria products vary widely in viability depending on how they were stored and how old they are, and some contain species that do not colonise a freshwater aquarium at all.

The model assumes a freshwater tank. Marine and brackish systems cycle differently, generally more slowly, with different bacterial species and additional considerations around live rock, and the factors here should not be applied to them without adjustment.

Finally, and most importantly: your test kit is the authority, not this page. The estimate tells you roughly when to expect the cycle to finish so you can plan; it cannot tell you the tank is safe. Add fish when the 24-hour ammonia test comes back clean, whatever the calculated day count says.

Related Calculators

The Aquarium Bioload Fish Stocking Capacity Calculator decides how much livestock the new filter can support once the cycle completes, which is the question that immediately follows this one. The Aquarium Volume Calculator gives the real water volume — after substrate and hardscape — that every dose here is calculated against. The Aquarium CO2 Injection Calculator covers the next stage for a planted tank, where CO2 and pH interact with the same carbonate chemistry that governs cycling.

Related concepts

The Nitrogen Cycle

Two bacterial conversions in sequence: ammonia to nitrite, then nitrite to nitrate. The second population grows more slowly than the first, which is why nitrite lingers long after ammonia has cleared.

Fishless Cycling

Establishing the bacterial colony by dosing pure ammonia rather than by adding fish. It is faster, because no water changes are needed, and it exposes nothing living to ammonia or nitrite.

Seeded Media

Filter material, gravel or squeezings from an established tank, carrying a live bacterial colony. It is the single most effective way to shorten a cycle, and considerably more reliable than bottled bacteria products.

Frequently Asked Questions

How long does it take to cycle a fish tank?+

The published range is three to eight weeks. A standard unseeded fishless cycle at 25 °C and pH 7.6 takes about 28 days. A warm, seeded, planted tank can finish in about a week; a cool, acidic, over-dosed fish-in cycle can take three months.

How do I know when my tank is cycled?+

Dose to 2 ppm ammonia and test 24 hours later. If both ammonia and nitrite read zero and nitrate is present, the tank is cycled. Nitrate appearing is the confirmation — a tank reading zero on all three is not cycled, it is a tank where nothing has happened.

Does temperature speed up cycling?+

Substantially. Nitrifying bacteria roughly double their rate for every 10 °C increase, and raising a tank from 25 to 30 °C saves around eight days on a four-week cycle. Turning the heater up for a month is the cheapest acceleration available.

Why is my nitrite stuck at high levels?+

Because nitrite-oxidising bacteria are slower-growing than ammonia oxidisers and account for roughly the back 60% of the cycle. A tank that clears ammonia in two weeks and then sits at high nitrite for another three is behaving normally, not stalling.

Does bottled bacteria actually work?+

It helps, cutting a cycle by roughly a third in this model, but it is not instant despite the marketing. Products vary widely in viability depending on age and storage, and some contain species that never colonise a freshwater aquarium. Media from an established filter is considerably more reliable.

How much ammonia should I dose?+

Around 2 ppm, and no more. Above 4 ppm the nitrite-oxidising bacteria are actively inhibited and the second half of the cycle stalls, so over-dosing makes the process slower rather than faster. About 1 mL of standard aquarium ammonia per 10 litres raises a tank roughly 1 ppm.

Why does low pH stall a cycle?+

Nitrifying bacteria need carbonate as a carbon source and work poorly in acidic water. Below pH 7.0 the process slows noticeably; below 6.5 it can stop altogether. A small carbonate addition to bring pH into the mid-sevens usually restarts a stalled cycle.

Should I change water while cycling?+

Not during a fishless cycle unless nitrite goes off the chart — water changes remove the ammonia and nitrite the bacteria are growing on. During a fish-in cycle you must change water to protect the fish, which is precisely why fish-in cycles take about 30% longer.

Formula

Cycling time model

Estimated days = 28 x temperature x pH x seeding x ammonia x method x plants x turnover
  (floored at 7 days)

Temperature  2^((25 - T) / 10), with penalties below 15 C and above 30 C
pH           1.00 above 7.4 | 1.15 at 7.0-7.4 | 1.50 at 6.5-7.0 | 2.20 below 6.5
Seeding      1.00 none | 0.65 bottled | 0.55 used gravel | 0.40 established media
Ammonia      1.00 at 1-3 ppm | 1.08 at 3-4 | 1.25 at 4-5 | 1.45 above 5 | 1.20 below 1
Method       1.00 fishless | 1.30 fish-in
Plants       0.88 with live plants

Ammonia phase = 40% of the total, nitrite phase = 60%

Step by step

How the calculation works

1The estimate places your tank inside the published three-to-eight-week range rather than repeating the range. The optimistic and pessimistic bounds are there because biology varies enough between identical setups that a single number would mislead.
2The two phases behave differently. Ammonia oxidisers establish first and account for about 40% of the wait; nitrite oxidisers are slower and account for the rest. A tank that clears ammonia quickly and then sits at high nitrite for weeks is normal, not stalled.
3Raising the temperature is the cheapest acceleration available — going from 25 to 30 °C saves around eight days on a four-week cycle. Dropping pH below 7.0 is the most common thing that stalls one.
4Dose to about 2 ppm and no higher. Above 4 ppm the nitrite-oxidising bacteria are actively inhibited, so over-dosing makes the cycle slower. Do not change water during a fishless cycle unless nitrite goes off the chart.
5The test that matters: dose to 2 ppm, wait 24 hours, and if both ammonia and nitrite read zero with nitrate present, the tank is cycled. Your test kit is the authority, not this estimate. The [aquarium bioload calculator](/calculators/aquarium-bioload-stocking-calculator) decides what the new filter can support.

Related guides

Learn more about this calculation

Guide

Fish Tank Cycling Time Calculator

Inputs

Results

Cycling time model

How the calculation works

Learn more about this calculation

Aquarium Volume Guide: Gallons, Liters, and Usable Water Capacity

How to Use Do The Calculation Calculators: A Practical Step-by-Step Guide

Aquarium Bioload & Stocking Calculator

Aquarium Volume Calculator

Aquarium CO2 Injection Calculator

Key facts

Fish Tank Cycling Time Calculator

Inputs

Results

Key output metrics

What Each Factor Multiplies the Cycle By

How to understand and use this calculator

Quick Answer — How Long Does It Take to Cycle a Tank?

How to Use This Calculator: A Standard Fishless Cycle

A Second Example: The Fastest and Slowest Cases

How to Tell When the Cycle Is Actually Done

Limitations

Related Calculators

Related concepts

Frequently Asked Questions

Cycling time model

How the calculation works

Learn more about this calculation

Aquarium Volume Guide: Gallons, Liters, and Usable Water Capacity

How to Use Do The Calculation Calculators: A Practical Step-by-Step Guide

Aquarium Bioload & Stocking Calculator

Aquarium Volume Calculator

Aquarium CO2 Injection Calculator

Key facts

Key output metrics

What Each Factor Multiplies the Cycle By

How to understand and use this calculator

Quick Answer — How Long Does It Take to Cycle a Tank?

How to Use This Calculator: A Standard Fishless Cycle

A Second Example: The Fastest and Slowest Cases

How to Tell When the Cycle Is Actually Done

Limitations

Related Calculators

Related concepts

Frequently Asked Questions