Time-critical
Ammonia poisoning in aquariums
PetPort EditorialLast reviewed
What is ammonia poisoning and how do I fix it?
Ammonia is produced constantly by fish waste and decaying food, and in a healthy tank filter bacteria convert it to something harmless. When that colony is missing or damaged, ammonia builds up and burns the gills. It is the commonest killer in home aquariums, and it is entirely preventable with testing.
General guidance for pet parents, not a diagnosis. Every animal is different — confirm anything that concerns you with your veterinarian.
Quick reference
| Safe ammonia level | Zero — there is no acceptable reading |
|---|---|
| Leading cause | New tank stocked before cycling, or filter media washed in tap water |
| Emergency action | 50% water change, dechlorinated and temperature-matched |
| Stop feeding for | 24–48 hours |
| Rinse filter media in | Tank water only, never tap water |
| Cycling a new tank takes | Four to six weeks before fish are added |
What is happening
Every aquarium needs a colony of bacteria living in the filter that converts ammonia to nitrite, and nitrite to relatively harmless nitrate. Establishing that colony is called cycling, and it takes weeks. A tank stocked before it is cycled — or one whose filter has been washed in tap water — has no such protection.
Signs to watch for
- Gasping at the surface, rapid gill movement, gills red or inflamed
- Red streaks on the body or fins, cloudy eyes, lethargy
- Several fish affected at once, often shortly after setting up or a filter clean
What you can do
- Do a 50% dechlorinated, temperature-matched water change immediately, and repeat daily until ammonia reads zero
- Stop feeding entirely for 24–48 hours — no food means no new ammonia
- Never rinse filter media in tap water; rinse it in water taken out of the tank
See a vet if
Fish continue to deteriorate after repeated water changes and confirmed zero ammonia. In practice, this is a system problem rather than a per-fish one — the priority is correcting the tank rather than finding a vet.
In India specifically
Two local practices cause most ammonia deaths here: fish sold and taken home the same day a new tank is filled, with no cycling period, and filter sponges rinsed under a running tap. Both are routine advice in many Indian aquarium shops. Municipal water is also chlorinated or chloraminated, so a dechlorinator is not optional for water changes.
Common questions
What is cycling a fish tank?
It is growing the bacterial colony in your filter that converts ammonia to nitrite and then to nitrate. It takes roughly four to six weeks and can be done without fish present by dosing a source of ammonia. Adding fish to an uncycled tank is the commonest reason beginner aquariums fail.
Why did my fish die after I cleaned the tank?
Almost certainly because the filter media was rinsed in chlorinated tap water, which kills the bacteria the tank depends on. Ammonia then rises within days. Always rinse media gently in a bucket of water removed from the tank, and never replace all the media at once.
Do ammonia-removing drops fix the problem?
Detoxifying products bind ammonia temporarily and are useful in an emergency, but they do not remove it or build the bacterial colony. Use them to buy time alongside water changes, not as a substitute for cycling the tank properly.
How often should I test my aquarium water?
Weekly for an established tank, and every day or two for a new one until it has cycled. A liquid test kit is more accurate than strips and lasts far longer. This is the single most useful piece of equipment a beginner can own.