Guide
Cycling a new aquarium
A new tank needs a few weeks for bacteria to settle that break down fish waste. Here is what happens in that time, what to measure and when the first animals can move in.
What happens during cycling?
Leftover food and waste turn into ammonium (NH₄⁺). At higher pH part of it becomes ammonia (NH₃), which is toxic even in small amounts. Bacteria in the filter and substrate convert ammonium into nitrite (NO₂⁻), also toxic. A second group of bacteria turns that into nitrate (NO₃⁻), which is harmless in normal amounts and removed by water changes and plants.
A new tank lacks these bacteria. They grow slowly, the second group more slowly than the first. So ammonium rises first, then nitrite, often sharply (the “nitrite spike”), and only after that nitrate. The tank is cycled when ammonium and nitrite stay at zero and nitrate is measurable.
How long does cycling take?
- Without help: usually 3 to 5 weeks, sometimes longer.
- With filter media or substrate from an established tank: often 2 to 4 weeks. This is the most reliable way to shorten cycling.
- With bottled bacteria: they can help, but they do not replace testing. Readings remain the only reliable indicator.
- Not before day 14: even if readings look good earlier, there are usually not yet enough bacteria for a full stock.
What to measure
During cycling, test ammonia, nitrite and nitrate every 2 to 3 days, plus pH and temperature. Drop tests are more accurate than test strips.
- Safe: ammonia and nitrite at most 0.1 mg/L, nitrate measurable (at least 5 mg/L).
- Cycled: two safe readings in a row, after 14 days at the earliest. Then the first hardy animals can move in, not the whole stock at once.
- Diatoms (brown coating) are normal in this phase and usually disappear on their own.
With or without animals?
Cycling without animals is the gentlest way, with plants and a little food or an ammonia source to feed the bacteria. The high readings then harm nobody.
If animals are already in the tank, act as soon as ammonia or nitrite rise: 30 to 50 % water changes, feed sparingly, no new animals. Fast-growing plants such as hornwort or waterweed absorb ammonium and relieve the tank in both cases.
The cycling assistant in the app
Aqualizer guides you through cycling: reminders to test, an assessment after every reading and the all-clear only when it really fits. Without animals water change reminders pause; with animals the app warns about nitrite straight away.
- Test reminders on your schedule
- Phase and estimated time left
- All-clear after two safe readings
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Cycling FAQ
How long does it take to cycle an aquarium?
Usually 3 to 5 weeks. With filter media from an established tank often only 2 to 4 weeks. The readings decide: ammonia and nitrite must be at most 0.1 mg/L in two readings in a row, after 14 days at the earliest.
When can I add the first fish?
When ammonia and nitrite stay at zero and nitrate is measurable. Start with a few hardy animals and increase the stock over several weeks. Sensitive species such as otocinclus or crystal shrimp only go into a tank that has been running for months.
What is the nitrite spike?
The period when nitrite rises sharply because the bacteria that break it down grow more slowly than the ones that produce it. Usually in the second or third week. Afterwards nitrite drops to zero within a few days.
Can I speed up cycling?
Best with filter media, substrate or plants from a healthy, established tank. Bottled bacteria can help. Plenty of fast-growing plants stabilise the tank further.
Do I need to change water during cycling?
Without animals not necessarily, the bacteria need the ammonia as food. With animals yes, as soon as ammonia or nitrite rise: 30 to 50 %. Before adding the first animals a large change is worthwhile to lower the accumulated nitrate.