Ammonia spike: the 24-hour plan when fish are gasping
Fish at the surface, red gills, a reading above zero — ammonia is the aquarium emergency that kills fastest and is easiest to fix. What to do in the next hour, the next day, and the next week.
The signs, and why speed matters
Ammonia burns gills. A fish in ammonia-laden water breathes fast and shallow, then moves to the surface where the oxygen is, then hangs there gasping. Gills look red or inflamed instead of pink; body colour fades; a fish may lie on the bottom, refuse food, or dart and flash as the burn itches. Snails climb to the waterline and shrimp sit still on the glass. Sometimes there is no sign at all until the morning a fish is dead, which is why a test kit is worth more than any medication.
Any ammonia reading above zero in a tank with fish is a problem; anything above 0.25 ppm is an emergency. The app treats 0.25 as the edge of the caution band and everything past it as an alert, and the toxicity rises sharply with pH and temperature — at pH 8 and 28 °C, 1 ppm of total ammonia is several times more poisonous than the same reading at pH 6.8 and 24 °C.
A newly set-up tank with fish added too early is the classic cause, but an established tank can spike too: a fish or snail died somewhere out of sight, a medication or a filter clean killed the bacteria, the power was off for a day, a bag of new fish brought a week’s waste with it, or a week of guests overfed the tank.
The first hour
1. Change 50 % of the water, now. Not 20 %, not “a big one at the weekend”. Halving the water halves the ammonia, and it is the only thing that works in minutes. Match the temperature to within a degree, dechlorinate the new water before it goes in, and pour it in gently against the glass or a plate.
2. Dose an ammonia detoxifier. A conditioner that binds ammonia (Seachem Prime, API Ammo Lock and similar) converts it to a form the fish can survive for a day or two while you fix the cause. It does not remove the ammonia and the test kit will still show it, but the fish stop dying. Dose for the full tank volume, and it can be re-dosed every 24–48 hours.
3. Add oxygen. An air stone, or the filter outlet turned up to churn the surface. Burned gills need every bit of oxygen the water can hold, and a 50 % change of fresh, cool-ish water helps here too.
4. Stop feeding. Nothing goes in the tank for 48 hours. Every flake is tomorrow’s ammonia.
In the app — log the reading and the verdict sheet flags the alert with the water change already calculated for your tank; Ask Finley what to do now sends your readings to the assistant and he walks you through this list against your actual tank. A checkup with a photo of the gills will tell you how much damage has been done.
The first day: find the cause
Change 25–50 % again in the evening if the reading is still above zero. Then, with the fish breathing easier, look for the source — because a spike with a cause you have not found comes back tonight.
- Count the fish and the snails. A body behind the filter or under the hardscape produces a stunning amount of ammonia. Check the intake.
- Check the filter is running and its media is intact. A filter that stopped for a day, or one whose media was rinsed in tap water, has lost its bacteria and the tank is effectively cycling again.
- Ask what changed this week. A new medication (many kill bacteria), a substrate vacuum that went deep, a lot of new fish, a new person feeding.
- In a tank under two months old, the cause is the cycle itself. The bacteria are not there yet. The treatment is the same — water changes to keep ammonia under 0.25 ppm and a detoxifier between them — but it lasts until the cycle completes, which means testing every day.
The week after
Test ammonia and nitrite every day until both have read zero for three days in a row. Nitrite is the second act: as the bacteria catch up on the ammonia they produce nitrite, which is nearly as toxic, and it spikes a few days after the ammonia falls. Water-change for it exactly the same way.
Feed lightly for the week — once a day, a minute’s worth. Do not add fish, do not add medication, do not clean the filter, and do not swap the substrate; everything you want is bacteria and all of those kill it. If you need to seed bacteria, a bottled nitrifying culture or a handful of media from a healthy tank’s filter helps; “ammonia removing” resin in the filter buys time but also starves the bacteria you are trying to grow.
Watch the fish. Gills go back to pink over a week or two; a fish that keeps gasping in water that now reads zero has gill damage and needs clean, well- oxygenated water and time. A checkup with a photo helps decide whether a secondary infection has set in on the damaged tissue.
In the app — the schedule advisor puts the tank on a test-every-two-days cadence while any toxin is above zero and relaxes it back to weekly when the readings clear, and the health score on the home screen stays red until they do. The follow-up check-in a few days after a checkup asks how the fish are doing and closes the case when they pull through.
Making sure it never happens again
A tank whose ammonia and nitrite read zero every week is a tank with enough bacteria for its fish. That balance breaks in three ways, and each has a habit that prevents it: never rinse filter media in tap water (a bucket of tank water, gently); quarantine and add fish a few at a time, so the bacteria can grow into the extra load; and test weekly, because every spike in this guide showed up on a test kit days before it showed up on a fish.