What should my water parameters look like at each stage of cycling?
When: Throughout cycling — use this to read your test results
Short answer: Cycling moves through four readable phases. First ammonia rises, then it falls as nitrite spikes, then nitrite falls as nitrate rises, and finally both ammonia and nitrite hold at zero while nitrate accumulates. The high nitrite spike in the middle is normal and the most nerve-wracking part — wait it out.
The details — the four phases
Phase 1 — Ammonia rise (roughly days 1–7). Ammonia climbs to your 2–4 ppm target. Nitrite and nitrate are still 0. The first bacteria (Nitrosomonas) are multiplying but too few to show results yet.
Phase 2 — Nitrite spike (roughly days 7–21). Ammonia starts falling as the first bacteria catch up. Nitrite now rises — sometimes to 2–5+ ppm — because the second bacteria (Nitrobacter) haven't scaled yet. Nitrate begins to appear in traces. pH may dip slightly because nitrification is acidifying. This is the phase people panic in. Don't.
Phase 3 — Nitrite decline (roughly days 14–35). The second bacteria colony is now big enough to consume nitrite as fast as it's made. Ammonia is processed within hours of dosing, nitrite falls toward 0, and nitrate climbs (10–40+ ppm).
Phase 4 — Complete. A ~2 ppm ammonia dose is converted to 0 ammonia and 0 nitrite within 24 hours. Nitrate is now high — do a large water change before adding livestock.
The numbers
| Phase | Ammonia | Nitrite | Nitrate |
|---|---|---|---|
| 1 — Ammonia rise | Rising → 2–4 ppm | 0 | 0 |
| 2 — Nitrite spike | Falling | Rising → 2–5+ ppm | Trace |
| 3 — Nitrite decline | 0 (fast) | Falling → 0 | Rising 10–40 ppm |
| 4 — Complete | 0 in 24 h | 0 in 24 h | High — water change before livestock |
Common mistakes
- Panicking at the nitrite spike and doing water changes. That just prolongs the cycle by removing what the bacteria are feeding on. Only intervene if nitrite exceeds ~5 ppm.
- Testing ammonia only. You can't tell phase 2 from phase 3 without also testing nitrite and nitrate.
When to worry
- Normal: Nitrite pinned high for a week or two in phase 2. Longest-feeling, most-normal part of cycling.
- Worry: Nitrite above 5 ppm — at that level it inhibits both bacteria and stalls the cycle. Do a partial water change to bring it under 5.
What's next
Run the finish-line test in is-my-tank-done-cycling.
- Nitrite above 5 ppm (inhibits bacteria — partial water change)
- Reef Tank Cycling Research (internal)