Ammonia spike: the first 24 hours
Ammonia above zero is the only reading in this hobby that justifies dropping everything. Here is the order of operations.
Ammonia (NH₃/NH₄⁺) is produced continuously by fish, by decaying food, and by anything organic in the tank. In a cycled tank it is consumed within hours. A reading of 0.25 ppm that used to be 0 ppm is a signal that consumption has fallen behind production — not that the tank is already dying.
Step 1 — confirm the reading
Retest before you act. Test strips and contaminated vials produce false positives often enough that a single reading is not evidence. Rinse the tube in tank water, use a fresh strip, and check the expiry date on the reagent. If the second test agrees, continue.
Step 2 — water change, sized to the reading
| Ammonia | Action | Notes |
|---|---|---|
| 0.25 ppm | 25–30% water change | Retest in 12 hours. Often a filter or feeding issue. |
| 0.5 ppm | 50% water change | Stop feeding for 48 hours. Add an air stone. |
| 1.0 ppm | 50% now, 30–50% again in 6–12 hours | Consider a temporary move of livestock. |
| 2.0 ppm + | 50% now, then daily 30–50% | This is an emergency. Ammonia binds gills. |
A cold water change during a crisis adds a second stressor. Match the temperature within a degree or two, dose the dechlorinator for the full tank volume (not just the replaced volume), and pour gently — a 50% change that stirs the substrate can release more ammonia than it removes.
Step 3 — do not clean the filter
The instinct is to clean everything. The filter is the thing currently keeping you alive. Do not rinse media under tap water, do not replace cartridges, and do not “deep clean” the substrate during an ammonia spike. You would be removing the bacteria that are already struggling.
Step 4 — stop feeding, keep oxygen up
Fish survive several days without food far more comfortably than they survive ammonia. Stop feeding for 48 hours — this removes the largest new ammonia input. Point a wavemaker or air stone at the surface: ammonia toxicity rises when pH and temperature are high, and oxygen helps the animals cope. Do not raise the pH during a spike; it converts ammonium into the more toxic free ammonia.
Step 5 — find the cause, then log it
- Overfeeding, or food that sank and was never eaten — the most common cause by a wide margin.
- A dead fish, snail or shrimp hidden behind hardscape.
- Filter media replaced, or rinsed in tap water, in the last two weeks.
- Stocking added too quickly for the bacteria to catch up.
- Medication that suppressed the biofilter — many antibacterials do.
Then write it down. Log the reading, the water change volume, and the suspected cause in the water test log. Ammonia spikes repeat in the same tank for the same reason more often than they happen once — and the second one is much easier to catch if the first one is documented.
Frequently asked
Is 0.25 ppm ammonia dangerous?
Do ammonia “locks” or detoxifiers work?
Should I add bottled bacteria?
How to cycle a new aquarium (fishless, day by day)
What actually happens during the nitrogen cycle, how long each stage takes, the readings that tell you where you are, and when it is safe to add fish.
Read →How often should you change aquarium water?
Interval and volume by tank size and stocking — plus the nitrate maths that turns “about 25% weekly” into a schedule you can defend with your own numbers.
Read →Stop guessing what your tank did last week
Log a test in twenty seconds, get reminded before maintenance is overdue, and keep a history you can actually read back. Free for one tank — no account, no subscription.
Open the App Store link →