Why Isn’t Nitrite Dropping?
Written 2026-09-29
Nitrite-oxidizing bacteria (Nitrobacter) simply grow slower than ammonia-oxidizing bacteria (Nitrosomonas), so it’s normal for nitrite to stay up for a while after ammonia has already dropped during cycling. In our simulator, dosing ammonia once on day 1 with no fish in the tank keeps both readings above zero until day 33. But if nitrite genuinely won’t drop, the usual cause is one of two things: you added more fish to an already-cycled tank before the colony caught up with the new load, or you’ve been doing water changes with unchlorinated tap water, wearing the colony down every week.
All the runs below use a 60cm tank (about 59L / 15.5gal), a large sponge filter, low light for 8 hours a day, and a heater set to 25°C (77°F). "Cycled" means the day ammonia and nitrite both drop below 0.05mg/L N (about 0.06ppm NH4 and 0.16ppm NO2 on a hobby test kit) and stay there. The nitrite safety line is 0.25mg/L N, about 0.8ppm on a test kit.
Nitrite-eating bacteria are built to grow slower
In this simulator’s model, the ammonia-oxidizing colony doubles roughly every 27 hours, while the nitrite-oxidizing colony doubles roughly every 35 hours and starts from a smaller seed population. Dosing ammonia once on day 1 with no fish shows this clearly: ammonia peaks at 1.03ppm (test-kit units) and comes down first, while nitrite peaks higher, at 1.95ppm, and lingers until day 33 before both finally clear. That lag is the model working as intended, not a malfunction.
If you added fish to an already-cycled tank
Dosing a bacteria starter on day 1 and adding 6 neon tetras on day 22 finishes cycling by day 29, and all six fish survive. Add 6 more neon tetras a week later, on day 36, and the colony hasn’t grown enough for the new load yet: nitrite spikes back up to 1.35ppm (about 1.6x the safety line) and doesn’t clear again until day 46, though all twelve fish survive. Add 10 fish instead of 6 on day 36, and nitrite peaks at 2.86ppm (about 3.5x the safety line) — fish start dying on day 42, and by day 48 only 7 of 16 survive. The bigger the addition, the longer the re-cycle takes and the higher the risk.
If you’ve been skipping dechlorinator
Take the same tank (starter on day 1, 6 neon tetras on day 3) and do weekly 25% water changes with unchlorinated tap water starting day 8: the colony gets knocked back every week, and nitrite doesn’t clear until day 76. Bump the water change to 40% and the colony never gets a chance to recover — nitrite still hasn’t fully cleared after 90 days, hovering around a peak of 0.31ppm (below the danger line, so it isn’t lethal, but it also never actually finishes). Run the same 40% weekly change with dechlorinator added, and nitrite barely registers at all, peaking at 0.04ppm. The only difference between those two runs is whether the water was dechlorinated.
| Setup | Nitrite peak | Cycled | Survived |
|---|---|---|---|
| Ammonia dosed once, no fish | 1.95ppm | Day 33 | n/a |
| Starter + 6 tetras on day 22 | 0.25ppm | Day 29 | 6/6 |
| + 6 more tetras on day 36 | 1.35ppm | Day 46 (re-spike) | 12/12 |
| + 10 more tetras on day 36 | 2.86ppm | Re-spike, deaths from day 42 | 7/16 |
| Weekly 25% change, no dechlorinator | 0.20ppm | Day 76 | 6/6 |
| Weekly 40% change, no dechlorinator | 0.31ppm | Never clears in 90 days | 6/6 |
| Weekly 40% change, with dechlorinator | 0.04ppm | Barely rises | 6/6 |
The takeaway: nitrite dropping later than ammonia is completely normal. If it genuinely won’t drop after weeks, the faster check is whether you’ve recently added fish or increased feeding, and whether you’re dechlorinating every single water change without exception.
The button below runs the tank from this article that got weekly 40% water changes with unchlorinated tap water.
The numbers in this guide come from the Water Kiwoogi simulator model; real tanks can differ.