Cycling your tank is the most important step before adding fish. It builds beneficial bacteria that converts harmful ammonia into safe nitrates.
Goal: your tank should process a measured ammonia dose to 0 ammonia / 0 nitrite within 24 hours before fish are added.
Before You Start Cycling
- Run your filter and heater continuously (target 76-80°F for faster bacterial growth).
- Dechlorinate all tap water before it enters the tank.
- Use a liquid test kit for ammonia, nitrite, and nitrate.
- Do not replace filter media during cycling unless it is physically falling apart.
What is the Nitrogen Cycle?
Fish produce ammonia through their gills and waste. Ammonia is toxic. The nitrogen cycle uses beneficial bacteria to convert:
- Ammonia (toxic) →
- Nitrites (still toxic) →
- Nitrates (safe at low levels)
Fishless Cycling Method (Recommended)
1 Add Ammonia Source
Add pure ammonia or a small amount of fish food to start the cycle. Aim for about 1-2 ppm ammonia in nano tanks.
2 Test Water Every Few Days
Use a test kit to monitor ammonia, nitrite, and nitrate. Expect ammonia to rise first, then drop as nitrite appears.
3 Wait for Nitrites
After ammonia drops, nitrites will spike. This shows the second colony of bacteria is forming.
4 Nitrates Appear
When nitrites drop and nitrates appear (0 ammonia, 0 nitrites), your tank is cycled!
5 Confirm with a 24-Hour Test
Dose ammonia back to about 1 ppm. If both ammonia and nitrite return to 0 within 24 hours, your cycle is established.
Typical Nano Tank Cycling Timeline
- Week 1: Ammonia rises, nitrite stays near zero.
- Week 2-3: Ammonia falls, nitrite spikes.
- Week 3-5: Nitrite falls, nitrate rises.
- Week 4-6: Tank processes 1 ppm ammonia in 24 hours.
How Long Does It Take?
Typically 4-6 weeks. Some tanks cycle faster with seeded media; others take longer if temperature is low or testing is inconsistent.
Fish-In Cycling (Only if You Already Have Fish)
Fish-in cycling is riskier and should only be used when fish are already in the tank and cannot be rehomed.
- Feed lightly once per day.
- Test ammonia and nitrite daily.
- Do a 30-50% water change any day ammonia or nitrite exceeds 0.25 ppm.
- Use dechlorinator every water change.
Continue this routine until tests consistently read 0 ammonia and 0 nitrite.
When Can I Add Fish?
Once your test shows 0 ammonia, 0 nitrite, and some nitrate (about 5-20 ppm), your tank is ready for fish.
- Do a large water change (50-70%) before stocking to reduce nitrate.
- Add livestock slowly over 2-3 weeks, not all at once.
- Retest 24 hours after each new livestock addition.
Common Cycling Mistakes
- Adding fish before ammonia and nitrite both reach zero.
- Overdosing ammonia above 4 ppm, which can stall bacteria growth.
- Turning off the filter overnight.
- Rinsing filter media under chlorinated tap water.
- Changing too many variables at once when troubleshooting.
Quick Troubleshooting
Ammonia Is Not Dropping
Verify your test kit, increase temperature slightly, and confirm dechlorinator is being used on all new water.
Nitrite Is Stuck High for Weeks
This is common in nano tanks. Keep dosing small ammonia amounts, maintain stable temperature, and wait for the second bacterial colony to catch up.
No Nitrate Reading
Shake nitrate reagent bottles thoroughly and test again. Some kits require vigorous shaking to produce accurate nitrate results.