mg/L
Ammonia
What is it
Ammonia (NH₃) is the first by-product of organic decomposition in an aquarium. It comes from fish waste, uneaten food, decomposing leaves, chloramine-treated tap water and dead organics. In water it occurs in two chemically balanced forms: toxic, un-ionised free ammonia (NH₃) and the almost harmless ammonium ion (NH₄⁺).
The balance between the forms is closely tied to pH and temperature. In acidic water (pH < 7), almost all ammonia converts to harmless ammonium. In alkaline water (pH > 7.5) and at high temperature, the toxic free-ammonia percentage rises exponentially.
As with nitrite, in an aquarium with a mature, working biological filter, ammonia must always be undetectable (0 mg/L), because Nitrosomonas bacteria consume it as soon as it is produced.
Why it matters
Free ammonia (NH₃) is the most lethal compound that can form in an aquarium. It is a powerful corrosive that literally burns fish mucus membranes and gills from outside, then enters the bloodstream and damages the liver, kidneys and brain.
It is the leading cause of new tank syndrome, meaning mass mortality in newly set-up aquariums, and of filter crashes caused by excessive cleaning, antibiotics or severe overfeeding.
In tanks with active soil that lowers pH to 6.0-6.5, ammonia released from the nitrogen-rich substrate in the first weeks is mainly ammonium, making maturation less risky; however, a sudden pH rise can instantly turn it into lethal ammonia.
Ideal ranges
Interactions with other parameters
Ammonia is completely dependent on pH and temperature: raising either makes it exponentially more toxic. It is the first vital link in the nitrogen cycle. Chloramine-treated tap water and dead organics can also add ammonia unexpectedly.
| Tank type | Min | Max | Unit |
|---|---|---|---|
| Tropical community | 0 | 0 | mg/L |
| Planted high-tech | 0 | 0 | mg/L |
| Planted low-tech | 0 | 0 | mg/L |
| Shrimp tank | 0 | 0 | mg/L |
Out of range: what happens
Acute poisoning causes fish to jump from the water, swim erratically into equipment, show red or bleeding gills, excess mucus that makes them look cloudy, total lethargy and death within hours.
Chronic poisoning from low but persistent levels causes permanent gill damage, extreme susceptibility to opportunistic diseases such as Ich and Fin Rot, and irreversible reductions in growth and life expectancy.
Environmental effects include uncontrolled blooms of single-celled algae, green water or cyanobacteria, especially under strong light. Ammonia is a powerful algae trigger.
Common Myths
- Ammonia is only a cycling problem; filter damage or overstocking can bring it back anytime, and even 0.05 ppm can cause chronic gill and liver damage.
- Detoxifier replaces water changes; it only reduces risk while the system catches up.
Measurement and adjustment
How to measure
Use a liquid reagent test such as API Ammonia or Seachem MultiTest. Most tests using Nessler or salicylate measure total ammonia (NH₃ + NH₄⁺), so determine the truly toxic free NH₃ by relating the result to pH and temperature with a chart.
Continuous monitors such as Seachem Ammonia Alert change colour in real time and react only to toxic NH₃. They are useful preventive safety tools to leave permanently in the tank.
In a mature, stable tank ammonia is tested rarely, but it becomes the vital test during cycling, after filter trouble or after a power outage, until readings return to zero.
How to adjust
For an immediate emergency (toxic NH₃ > 0.05 mg/L), change 50-70% of the water with dechlorinated water at the same temperature. Use a conditioner that temporarily detoxifies ammonia, such as Seachem Prime, for 24-48 hours.
In the short term, stop feeding immediately because healthy fish can fast for days, add high-quality liquid nitrifying bacteria, and place zeolite in the filter to absorb ammonia actively. Add aeration and protect filter media.
In the long term, do not overstock, do not clean biological media under tap water because chlorine kills bacteria, and do not replace all filter media at once. Increase biological media such as ceramic rings or Matrix; quarantine tanks need seeded media or very light feeding to avoid spikes.
Pro Tips
- Keep a bottle of Seachem Prime available: a fivefold emergency dose neutralises lethal ammonia within minutes and can save the tank during a filter crash.
- Watch pH: if you have 1 ppm ammonia at pH 6.0 it is not serious because it is ammonium, but if a water change raises pH to 8.0, the same tank becomes lethal in 10 seconds. After a power outage, check ammonia because oxygen-starved filter bacteria may have suffered; quarantine tanks need seeded media or very light feeding to avoid spikes.