What is the Nitrogen Cycle?
The nitrogen cycle is the single most important concept in fishkeeping. Without understanding it, you will kill fish. It is the biological process by which beneficial bacteria in your filter convert toxic Ammonia (from fish waste) → into slightly less toxic Nitrites → into relatively harmless Nitrates (removed by water changes).
Why You MUST Cycle Before Adding Fish
“New Tank Syndrome” — adding fish to an uncycled tank — is the number one cause of fish death in the hobby. Without established bacteria colonies, ammonia from the very first feed will spike to lethal levels within 24–48 hours. The fish will gasp at the surface, develop red/inflamed gills, and die.
Fishless Cycling: Step by Step
- Set up the tank completely: filter running, heater set to 26–28°C (bacteria grow faster in warmth), dechlorinated water.
- Add ammonia source: Use pure household ammonia (Dr Tim’s or pure janitorial ammonia with no surfactants) to raise the level to 2–4ppm.
- Test daily with a liquid test kit (API Master Kit). You will see ammonia rise, then slowly drop as Nitrites appear.
- Wait for Nitrites to spike and then drop to zero. This typically takes 4–6 weeks.
- When both Ammonia AND Nitrites read 0ppm within 24 hours of dosing, the cycle is complete. Do a large (70–80%) water change to reduce Nitrates, and add your first fish.
Speeding Up the Process
Using mature filter media from an established tank, or bottled bacteria products (like Seachem Stability), can significantly accelerate cycling. Some experienced fishkeepers can achieve a cycle in 2 weeks using these methods.
