Cycle and parameters
Ammonia is at zero but nitrite will not come down
Zero ammonia with high nitrite means the first half of your biological filter is working and the second half has not caught up. This is normal, it is the longest part of cycling, and the bacteria involved are genuinely slow. Water changes protect the fish without slowing the process — nothing else you add is likely to shorten it much.
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Test these first
Before anything goes into the tank and before anything comes out of it, get these numbers. They cost fifteen minutes and they decide everything that follows.
- Nitrite — nitrite ion, the intermediate product between ammonia and nitrate
- Ammonia — total ammonia — the ammonium ion plus free ammonia, added together
- pH and KH — acidity of the water, plus the carbonate buffer that resists changing it
- Temperature — the temperature of the water itself — not of the room the tank sits in
What is usually behind it
| Cause | How to confirm | Time to resolve |
|---|---|---|
| The second bacterial group is simply still growing | Ammonia has been at zero for at least several days, nitrite is high, and nitrate has begun to appear. The tank is between two and six weeks old. | One to three more weeks in most tanks |
| pH has drifted low and slowed nitrification | pH reads in the low sixes and KH is at or near the bottom of its test range. This often develops quietly over several weeks in soft water. | Several days after KH is back up, then the cycle resumes at normal pace |
| Water too cold for the bacteria to work quickly | A thermometer in the water reads below about 70 °F. Room temperature and water temperature are not the same thing, and a room monitor cannot answer this question. | Cycle speed improves within days of a stable temperature |
| Excessive water changes removing the food supply | You are changing a large volume daily and nitrite briefly hits zero after each change, then rises again. In a fishless cycle this is a real cause of a long stall. | Progress resumes within a week |
| Filter media disturbed part-way through the cycle | The stall began after a cartridge change, a media rinse, or the filter being off for several hours. | Two to three weeks to recover the lost ground |
The second bacterial group is simply still growing
How to confirmAmmonia has been at zero for at least several days, nitrite is high, and nitrate has begun to appear. The tank is between two and six weeks old.
What to doChange 30 to 50% of the water whenever nitrite goes above 0.25 ppm with fish present, keep feeding light, and leave the filter alone entirely. Test at the same time each day so the trend is readable.
Time to resolve: One to three more weeks in most tanks
pH has drifted low and slowed nitrification
How to confirmpH reads in the low sixes and KH is at or near the bottom of its test range. This often develops quietly over several weeks in soft water.
What to doRestore buffer the ordinary way, by changing water regularly with water that has some KH in it. Do not reach for a pH-up product — it moves the number without adding what holds it, and the tank comes straight back.
Time to resolve: Several days after KH is back up, then the cycle resumes at normal pace
Water too cold for the bacteria to work quickly
How to confirmA thermometer in the water reads below about 70 °F. Room temperature and water temperature are not the same thing, and a room monitor cannot answer this question.
What to doBring the temperature into the range appropriate for your species and hold it steady. Unplug the heater before any water change and leave it unplugged until the glass is submerged again.
Time to resolve: Cycle speed improves within days of a stable temperature
Excessive water changes removing the food supply
How to confirmYou are changing a large volume daily and nitrite briefly hits zero after each change, then rises again. In a fishless cycle this is a real cause of a long stall.
What to doIn a fishless cycle, stop changing water and let nitrite sit high — nothing is being harmed. In a fish-in cycle, change on the readings rather than on a routine, and use the smallest change that gets nitrite under 0.25 ppm.
Time to resolve: Progress resumes within a week
Filter media disturbed part-way through the cycle
How to confirmThe stall began after a cartridge change, a media rinse, or the filter being off for several hours.
What to doLeave everything in place from now on. If you replaced a cartridge, keep the old one in the filter body alongside the new one until the cycle finishes, however unappealing it looks.
Time to resolve: Two to three weeks to recover the lost ground
What not to do
These are the moves that make this particular problem worse, and they are all common. Most of them come from wanting to do something substantial when the right answer is something small.
- Do not restart the tank. Tearing it down and starting again puts you back at week one with the same bacteria you would have had anyway.
- Do not rinse or replace filter media to "help it along". Every colony you need is on that media.
- Do not add a second and third bacteria product on top of the first. If the first did not seat, more bottles will not change the biology.
- Do not stock more fish while nitrite is detectable, whatever the shop says about hardy species.
When this stops being a water question
Nitrite interferes with oxygen transport in the blood, so fish here often breathe hard and hang near the outflow even in a well-aerated tank. Change water now and add surface agitation. If a fish remains distressed once nitrite reads zero, that is beyond water chemistry and belongs with an aquatic veterinarian.
Keeping it from coming back
Nothing prevents the nitrite stage — every tank passes through it. What prevents the stall is patience with a written log: test at the same time daily, write the two numbers down, and only intervene when the log says you need to. Most of the tanks that take three months to cycle are tanks where something was changed every few days in the hope of speeding it up.
Reference for this page
Parameters
Equipment involved
- Test Kitsknowing the number instead of guessing it
- Filter Media & Cartridgeswhat actually does the filtering, and when it gets replaced
- Water Conditioners & Additivesmaking tap water safe to put in a tank
Related problems
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My new tank is reading ammonia and there are already fish in it
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Published sources
- USGS Water Science School — Nitrogen and Water
- USGS Water Science School — pH and Water
- US EPA — Aquatic Life Criteria and Methods for Toxics
Published figures on this page come from the documents above. Where a value depends on species, pH or temperature, that is stated rather than averaged into a single number.