How do I know when my tank is done cycling?
When: At the end of cycling — this is the gate you must pass before livestock
Short answer: Dose the tank to about 2 ppm ammonia, wait 24 hours, and test again. If both ammonia and nitrite read essentially 0 (below 0.2 ppm) the next day, the cycle is complete — your bacteria can clear a full day's waste overnight. If either is still detectable, you're not done; keep going and retest. This 24-hour test is the gate for leaving the cycling stage.
The details — the confirmation test
This is the one test that actually matters, and it's simple:
- Dose ammonium chloride to about 2 ppm ammonia (roughly 4 drops/gallon of Dr. Tim's).
- Wait 24 hours. Don't touch anything.
- Test ammonia and nitrite.
- Pass: both below ~0.2 ppm → the tank can safely process 2 ppm of ammonia in a day, which is enough for an initial fish load. Cycle complete.
- Fail: either reading still elevated → the colony isn't big enough yet. Wait a few days and repeat.
Why 24 hours and why both: nitrite is as toxic as ammonia, and beginners often stop at "ammonia is 0" while nitrite is still high. The tank isn't safe until both clear on the same test.
The numbers
| Step | Value |
|---|---|
| Test dose | ~2 ppm ammonia |
| Wait | 24 hours |
| Pass — ammonia | < 0.2 ppm |
| Pass — nitrite | < 0.2 ppm |
| Nitrate | Will be high — plan a big water change before livestock |
Common mistakes
- Declaring victory on ammonia alone. Test nitrite too. This is the single most common false finish.
- Skipping the 24-hour wait. Testing an hour after dosing tells you nothing about the colony's daily capacity.
- Adding livestock the instant it passes, without a water change. Nitrate is high now — see first-steps-after-cycling.
When to worry
- Normal: Needing to repeat the test a couple of times before it passes.
- Worry: Ammonia clears but nitrite never does after weeks — the second bacteria stalled (often high nitrite >5 ppm inhibiting them, or crashed pH). Do a water change to reset and keep the pH above 7.0.
What's next
You've passed the gate. Do your first big water change and add a clean-up crew: first-steps-after-cycling.
- Any leftover ammonia or nitrite 24h after the test dose = not done, do not add livestock
- Reef Tank Cycling Research (internal)
- Dr. Tim's — Fishless Cycling