When is a tank considered cycled?

A tank is cycled when ammonia and nitrite both read 0 ppm while nitrate is present and rising. That means your bacteria can convert toxic ammonia all the way to relatively harmless nitrate within 24 hours. The tracker flags this state as Complete.

What are the phases of the nitrogen cycle?

First ammonia rises as waste accumulates. Then ammonia-oxidising bacteria appear and nitrite climbs while ammonia falls. Finally nitrite-oxidising bacteria establish, nitrite drops to zero, and nitrate accumulates. The tool labels which phase your latest reading matches.

How long does cycling usually take?

A fishless cycle typically takes four to eight weeks at normal room temperature. Warmer water, an established filter media seed, or bottled bacteria can shorten it considerably. The estimate here is a rough guide from your own trend, not a guarantee.

Should I do water changes while cycling?

During a fishless cycle you generally avoid large water changes so the bacteria have a steady food source, unless ammonia or nitrite spike very high. If fish are present, change water whenever ammonia or nitrite exceed safe levels to protect them.

Why is my nitrate so high at the end?

Nitrate is the end product of the cycle and is not removed by bacteria. Once cycled, regular partial water changes keep nitrate in a safe range, typically under 20 to 40 ppm for most freshwater community tanks.

What is the Aquarium Nitrogen Cycle Tracker?

Log daily test readings for ammonia, nitrite, and nitrate to see which phase of the nitrogen cycle your new aquarium is in and estimate days to a completed cycle. Readings save in your browser between test days. It runs free in your browser on Gera Tools, with nothing uploaded.

Aquarium Nitrogen Cycle Tracker

Name: Aquarium Nitrogen Cycle Tracker
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Cycling a new aquarium means waiting for two colonies of beneficial bacteria to establish before adding fish. This tracker turns your daily test-kit readings into a clear picture of which phase you are in, whether the cycle has finished, and roughly how long is left.

How it works

The nitrogen cycle has three measurable phases, identified from the latest reading:

Ammonia phase : ammonia rising,  nitrite ~0, nitrate ~0
Nitrite phase : ammonia falling, nitrite rising
Complete      : ammonia = 0 AND nitrite = 0 AND nitrate > 0

The days-remaining estimate looks at how fast ammonia and nitrite are falling across your two most recent readings and projects when both should reach zero. A falling trend gives a finite estimate; a flat or rising trend means the cycle has not yet turned the corner.

What actually happens in each phase

Understanding the biology behind each phase helps you interpret what your test kit is telling you.

Ammonia phase. The moment you add fish food, pure ammonia, or a bacterial seed to a new tank, ammonia accumulates because there are no bacteria yet to consume it. You will see the ammonia reading climb from 0 toward 2–4 ppm (parts per million) while nitrite and nitrate remain near zero. This is normal — resist the urge to do massive water changes, which would dilute the ammonia that the first bacteria colony needs to feed on.

Nitrite phase. After roughly one to two weeks, Nitrosomonas and related ammonia-oxidising bacteria begin to establish. They convert ammonia into nitrite. Ammonia starts to fall and nitrite climbs sharply — sometimes to 5 ppm or above. This is the most toxic window of the cycle for any fish present, because nitrite blocks oxygen transport in fish blood (brown blood disease). If you have fish in the tank during this phase, monitor closely and perform partial water changes if nitrite rises above 1 ppm.

Nitrate phase and completion. The second colony, nitrite-oxidising bacteria such as Nitrospira, converts nitrite to nitrate. Nitrite drops toward zero, nitrate rises, and the tank is cycled. The tracker flags completion when ammonia = 0 AND nitrite = 0 AND nitrate is detectably above zero — meaning both colonies are active and the chain is working.

A worked example of a fishless cycle

For example: you add pure ammonia to reach roughly 2 ppm in a new tank. Day 1–7: ammonia holds between 2 and 3 ppm, nitrite at 0. Day 8: nitrite climbs to 0.5 ppm, ammonia starts falling. Day 14: ammonia near 0, nitrite at 3 ppm — the tracker shows Nitrite Phase. Day 21: nitrite falling to 0.5 ppm, nitrate rising to 10 ppm. Day 24: both ammonia and nitrite read 0, nitrate at 15 ppm — the tracker flags Complete.

This is an illustrative timeline. Cooler temperatures, a smaller bacterial seed, or low ammonia dosing can extend it to six to eight weeks. Warmer water (26–28 °C) and a filter media seed from an established tank can cut it to under two weeks.

Practical tips and common mistakes

Use a liquid reagent kit, not test strips. Strips are notoriously inaccurate at the low concentrations that matter near cycle completion. A liquid kit reading of 0 ppm is far more reliable.
Test at the same time each day. Ammonia and nitrite concentrations can fluctuate across the day with feeding and the light cycle; testing at a consistent time gives you a comparable trend.
Do not add fish during the nitrite spike. Nitrite toxicity is invisible and fast. Wait for two consecutive days of zero ammonia and zero nitrite before adding fish.
A spike then rapid drop is a good sign. If you see ammonia shoot up then fall sharply while nitrite starts rising, the first colony is established and working — you are on track.
Aeration matters. Nitrifying bacteria are strict aerobes; a well-oxygenated tank with good surface movement grows bacteria colonies faster.

Tips and notes

Use a liquid reagent test kit rather than strips for accuracy at low concentrations. Test at the same time each day for a consistent trend. Keep the water warm (around 26 to 28 C) and well-oxygenated to speed bacterial growth. All your readings stay in this browser, so you can log over weeks of cycling and revisit the trend at any time without an account.