Every fish produces ammonia through its gills and waste, and uneaten food and dead plant matter add more. Ammonia is toxic even in small amounts. In a healthy aquarium, bacteria living in the filter and substrate turn it into less harmful compounds. This process is the nitrogen cycle, and it is the single most important thing to understand as a fishkeeper.
The three stages
- Ammonia (NH₃/NH₄⁺) is released by fish, food and decay. Safe level: 0 ppm.
- Nitrite (NO₂⁻) is produced when Nitrosomonas-type bacteria break ammonia down. It is also toxic; it blocks oxygen transport in the blood. Safe level: 0 ppm.
- Nitrate (NO₃⁻) is produced when Nitrobacter/Nitrospira-type bacteria convert nitrite. It is far less toxic and is removed with water changes and plant growth. Keep it below 20–40 ppm in most freshwater tanks and much lower for sensitive species and reef tanks.
Why a new tank must cycle
A new tank has almost none of these bacteria. If you add a full load of fish on day one, ammonia and then nitrite rise quickly and fish get sick or die. This is often called new tank syndrome. Cycling means growing enough bacteria before the fish arrive.
How to cycle a tank without fish
- Set up the tank with filter and heater running (bacteria grow faster at 25–28 °C).
- Add an ammonia source: pure ammonium chloride dosed to about 2 ppm, or a pinch of fish food every day.
- Test ammonia, nitrite and nitrate every 2–3 days.
- First ammonia falls and nitrite rises. Later nitrite falls and nitrate appears.
- The cycle is complete when the tank can turn 2 ppm ammonia into 0 ammonia and 0 nitrite within 24 hours.
- Do a large water change to bring nitrate down, then add your first fish.
How long does it take?
Usually 3–6 weeks. It is slower in cold water, at very low pH (below about 6.5 the bacteria slow down sharply) and if the tap water contains chloramine that isn’t neutralized.
Signs that the cycle is complete
- Ammonia: 0 ppm
- Nitrite: 0 ppm
- Nitrate: above 0 and rising slowly between water changes
Keeping the cycle healthy
The bacteria live on surfaces, mostly in the filter. Protect them:
- Rinse filter media only in old tank water, never under the tap; chlorine kills the colony.
- Never replace all the filter media at once.
- Don’t switch the filter off for more than an hour or two.
- Some medications and antibiotics can damage the colony, so test ammonia daily during treatment.
- Add new fish gradually so the colony can keep up.
If ammonia or nitrite ever show up in an established tank, read what to do about ammonia. Fishtify tracks all three values against safe limits and shows where your tank is in the cycle.