Likely cause
Cycling a New Tank
A new tank needs several weeks to build up the beneficial bacteria that break down fish waste before it's safe to stock at normal levels. Test regularly for ammonia and nitrite, and add fish gradually.
Why this happens
- A tank always needs time to build up nitrifying bacteria before it can safely process fish waste — this isn't preventable, only managed.
- Cycling can stall if ammonia levels get too high for the bacteria colony to handle, or if the water is too cold for bacteria to grow efficiently.
What to do next
- Add an ammonia source (fish food, pure ammonia, or a few hardy fish) to feed the bacteria you're growing.
- Test ammonia and nitrite daily; nitrate rising while ammonia and nitrite fall to zero signals the cycle is progressing.
- Add a beneficial bacteria starter product to speed up the process significantly.
- Only add fish gradually, a few at a time, once ammonia and nitrite both read zero consistently.
Frequently asked questions
How long does it take to cycle a fish tank?
A fishless cycle typically takes 4-6 weeks without a bacteria starter product, or as little as 1-2 weeks with one.
Can I add fish before the tank is fully cycled?
It's not recommended — fish added to an uncycled tank are exposed to ammonia and nitrite spikes that can be fatal, known as new tank syndrome.