The nitrogen cycle: why does an aquarium need to "cycle"?

The most common cause of fish deaths in an aquarium is not disease but invisible poisons. Fish waste, leftover food and decaying plants release ammonia into the water. In nature this ammonia is diluted in an endless volume of water; in an aquarium it builds up. What makes it harmless are the beneficial bacteria in the filter and on surfaces. The establishment of this bacterial colony is called the nitrogen cycle, or "cycling" the aquarium.

The cycle has three steps

1. Ammonia (NH₃/NH₄⁺): Forms from fish waste and decaying food. Even a very small amount burns the gills.
2. Nitrite (NO₂⁻): The first group of bacteria converts ammonia into nitrite. Nitrite is also toxic; it prevents the blood from carrying oxygen.
3. Nitrate (NO₃⁻): The second group of bacteria converts nitrite into nitrate. Nitrate is far less harmful but it accumulates; it is removed through water changes and by plants.

In a cycled aquarium, ammonia and nitrite are always zero, and nitrate is kept below 40 mg/L.

Fishless cycling (the recommended method)

1. Set up the aquarium and run the filter and heater. If you use tap water, add a dechlorinator.
2. "Feed" the bacteria: a pinch of fish food or drops of pure ammonia (aiming for around 2 mg/L of ammonia).
3. Test ammonia, nitrite and nitrate every 2–3 days.
4. First ammonia rises, then nitrite rises as ammonia falls, and finally nitrite drops and nitrate appears.
5. If the tank can bring 2 mg/L of ammonia down to zero, and nitrite down to zero as well, within 24 hours, the aquarium is ready. This usually takes 4–6 weeks.
6. Do a large water change to lower nitrate and add fish a few at a time.

A filter sponge taken from a cycled aquarium or ready-made bacteria products can speed up the process.

Common mistakes

• Rinsing the filter in tap water: Chlorine kills the bacterial colony. Always rinse the filter sponge in water taken from the aquarium.
• Adding all the fish at once: The colony cannot keep up with the sudden load; ammonia spikes.
• Overfeeding: Uneaten food is a source of ammonia.
• "The water is clear, so it must be fine": Ammonia and nitrite are colorless; they can only be seen with a test kit.

Measure with the community

If you register your aquarium on the site and enter your measurements, you can follow the progress of the cycle on a chart. Your measurements during the cycling period also serve as an example for other hobbyists.