The short answer
A cycle that won’t get going is almost always missing one of the conditions your beneficial bacteria need. The usual culprits are no ammonia source, chlorine killing the bacteria, a pH that’s crashed too low, water that’s too cold, or simply not enough time. Work through those and the cycle almost always restarts.
The common blockers
Nitrifying bacteria need food, the right chemistry and warmth. If any is missing, the cycle stalls:
- No ammonia source. In a fishless cycle you must add an ammonia source (bottled ammonia or a food source) to feed the bacteria. No ammonia, no bacteria.
- Chlorine or chloramine from undechlorinated tap water kills the bacteria each time you add it. Always dechlorinate — see how to dechlorinate tap water.
- pH too low. Below about 6.5 nitrifying bacteria slow dramatically; below 6.0 they can stall entirely. A crashed pH stops the cycle cold.
- Water too cold. Bacteria establish fastest around 25–28°C. A cold tank cycles very slowly.
Diagnose with a test kit. A cycle shows a clear pattern: ammonia rises, then nitrite rises as ammonia falls, then nitrate appears as nitrite falls. If nothing moves for weeks, a liquid test kit tells you which stage is stuck.
How to restart a stuck cycle
- Confirm there’s an ammonia source feeding the bacteria.
- Check pH and raise it if it has crashed below 6.5 — the buffer may be exhausted.
- Dechlorinate everything you add, without exception.
- Warm the water to the mid-20s°C to speed bacterial growth.
- Seed the tank with mature filter media from an established tank, or a quality bacteria starter.
Give it time
Even a healthy cycle takes four to six weeks. Don’t keep resetting it with big water changes or by cleaning the filter — leave the bacteria to colonise. For the full step-by-step, see fishless cycling and how to cycle an aquarium, and browse the filter hub for media that hosts your bacteria.