Ammonia should always read 0 ppm in an established aquarium. Any measurable level means waste is being produced faster than the filter bacteria can process it, and fish start to suffer.
Why ammonia is dangerous
Ammonia burns the gills and skin and makes it hard for fish to breathe. Toxicity depends heavily on pH and temperature: at pH above 7 and in warm water, more of it is in the toxic free form (NH₃). The same test reading is therefore more dangerous in a hard-water or marine tank than in a soft, acidic one. Still, treat any reading above 0 as a warning.
Common causes
- A new tank that hasn’t finished cycling
- Too many fish added at once
- Overfeeding or uneaten food rotting on the bottom
- A dead fish, snail or decaying plants hidden in the tank
- Filter media rinsed under tap water or replaced all at once
- The filter being off for several hours, for example after a power cut
- Medications that harmed the bacteria colony
Symptoms in fish
- Gasping at the surface or rapid gill movement
- Red or inflamed gills
- Lethargy, loss of appetite, clamped fins
- Fish hiding or lying on the bottom
What to do, step by step
- Test again to rule out a reading error, and test nitrite too.
- Do a 30–50% water change with conditioned water at the same temperature.
- Dose a conditioner that binds ammonia temporarily; it buys time for the bacteria.
- Stop feeding for 24–48 hours. Fish can go several days without food; the ammonia can’t wait.
- Search for the source: dead livestock, rotting food, a clogged filter.
- Increase aeration with an air stone or more surface agitation.
- Test daily and repeat smaller water changes until ammonia stays at 0 for several days.
Preventing it
- Feed only what is eaten within two minutes.
- Increase stock slowly, a few fish at a time.
- Clean filter media in tank water and in rotation.
- Test weekly so a problem shows up before fish show symptoms.
Fishtify flags ammonia the moment you log it, shows the exact water change for your tank volume and reminds you to retest.