What Is the Nitrogen Cycle?
Fish produce ammonia (NH₃) through their waste and gills. Ammonia is toxic to fish even at low levels. The nitrogen cycle is the process by which beneficial bacteria convert ammonia → nitrite (NO₂) → nitrate (NO₃). Nitrate is far less harmful and is removed through water changes.
The bacteria that do this work — primarily Nitrosomonas and Nitrospira — live in your filter media, substrate and on every surface in your tank. Building up a large enough colony of these bacteria before adding fish is called "cycling the tank."
How Long Does Cycling Take?
A fishless cycle typically takes 4–8 weeks. You can speed this up with bottled bacteria products like Seachem Stability or API QuickStart, which can reduce the time to 1–2 weeks if used correctly.
The Fishless Cycle Method (Recommended)
- Set up your tank — fill with dechlorinated water, add filter, heater (set to 26–28°C), and substrate.
- Add an ammonia source — use pure ammonia (no surfactants) dosed to 2–4 ppm, OR add a pinch of fish food and let it decompose daily.
- Test every 2–3 days — use a liquid test kit to check ammonia, nitrite and nitrate levels.
- Wait for the spike and drop — ammonia will rise, then nitrite will rise as ammonia falls. When nitrite also falls and nitrate rises, your cycle is complete.
- Do a large water change (50–70%) before adding fish to dilute accumulated nitrates.
Signs Your Tank Is Cycled
- Ammonia: 0 ppm
- Nitrite: 0 ppm
- Nitrate: present (5–20 ppm is ideal)
Common Mistakes
Using tap water without dechlorinating: Chlorine kills the beneficial bacteria you are trying to grow. Always use a dechlorinator like Seachem Prime before adding tap water.
Cleaning the filter with tap water: Never rinse filter media under the tap. Squeeze it out in a bucket of tank water to preserve the bacteria.
Overfeeding during cycling: Extra food means extra ammonia, which can spike to levels that kill even the bacteria you are building up.
Speeding Up the Cycle
The fastest way to cycle a new tank is to use filter media or gravel from an already-established, healthy tank. This transfers live bacteria directly. Adding a bottle of Seachem Stability alongside this can cycle a tank in as little as 7 days.
