Ammonia in an aquarium: why it is the most dangerous
Updated on August 26, 2026 · ~6 min read
Ammonia in an aquarium must always read 0 ppm: it is the most toxic of the three nitrogen compounds and damages gills and nervous system within hours. Any value above 0 calls for an immediate water change, food suspended and a hunt for the cause — a dead fish, a badly washed filter, too much load.
Of all the values measured in an aquarium, ammonia is the only one with no range: the correct value is zero. Not "low", not "acceptable": zero. Any reading above zero is a problem in progress, not a margin to manage.
What it is and where it comes from
Ammonia is the first product of the decomposition of anything containing nitrogen. In an aquarium it comes from:
- the gills and faeces of the fish, which excrete it constantly;
- uneaten food, decomposing on the bottom;
- decaying plants and leaves;
- a dead animal left out of sight — the single most frequent cause of a sudden spike in a stable tank;
- substrate stirred up after months, releasing in one go what it had accumulated.
In a mature tank there is no trace of it, not because none is produced but because the nitrifying bacteria consume it as soon as it forms. A test at zero, in an established tank, measures the work of those bacteria more than the absence of waste.
Why it is the most dangerous of the three
Ammonia, nitrite and nitrate are the three steps of the nitrogen cycle, and toxicity falls along the chain. Ammonia is the first step and the worst: it crosses membranes, burns the epithelium of the gills, damages the nervous system and reduces the ability to take up oxygen. At concentrations a liquid kit reads as "the first step on the scale" — 0.25 ppm — prolonged exposure leaves permanent damage.
Then there is a complication that explains why the same number does not mean the same thing in two different tanks.
NH₃ and NH₄⁺: pH changes everything
In water ammonia exists in two forms in equilibrium: NH₃, gaseous and very toxic, and NH₄⁺, ionised and relatively harmless. Which of the two prevails is decided by pH and temperature: the more alkaline and the warmer the water, the higher the toxic NH₃ fraction.
In practice: 0.5 ppm of total ammonia at pH 6.5 and 24 °C (75 °F) is a modest problem, because nearly all of it is in the NH₄⁺ form. The same 0.5 ppm at pH 8.2 and 27 °C (81 °F) — typical marine conditions — is an emergency. Aquarium tests measure total ammonia, that is the sum of the two, and do not tell them apart.
Hence a practical and rather counter-intuitive consequence: with ammonia in the tank, do not raise pH. Anyone trying to "sort the parameters out" all at once, raising pH while ammonia is present, converts the harmless part into the toxic one in a single move.
What to do when the test detects it
1. A water change, immediately. 30-50%, same temperature, water treated with a dechlorinator. It is the only measure that lowers the concentration quickly, and it can be repeated daily for as long as needed.
2. Suspend feeding for 24-48 hours. Food is the main source of new ammonia, and no healthy adult fish suffers from a couple of fasting days.
3. Look for the cause, which is usually one of five: a dead animal out of sight; the filter washed under the tap (chlorine killed the bacteria); too many fish added at once; a medication or antibiotic that hit the useful bacteria too; the filter media all replaced at once instead of in stages.
4. Do not touch pH and do not add anything beyond possibly some bottled bacteria. In particular: no medications, no fertilisers, no new arrivals.
5. Measure again the next day. A single change rarely settles the matter, and you need to know whether the direction is right.
6. Help them breathe. More surface movement, and if there is CO₂ injection, shut it off. It does not remove ammonia from the blood, but a fish with damaged gills needs all the oxygenation it can get. For the same reason, if the temperature is high you can bring it down a degree or two — colder water holds more oxygen, and it also reduces the toxic NH₃ fraction — but gradually, never with a change of cold water.
"Ammonia removers" do not replace a water change. Some bind it temporarily into a less available form, and in a real emergency they can buy a few hours. But they do not eliminate it, they interfere with several tests — after using them the number you read is no longer comparable with the one before — and above all they give the feeling that something has been solved. The water change remains the first measure, always.
How to measure it, and how it goes wrong
The liquid ammonia kit is reliable but has three traps:
- The colour scale is hard to read at low values: the step between 0 and 0.25 ppm is one shade of yellow-green. Read it in daylight, against a white background, looking down rather than from the side.
- Reagents expire. A test opened two years ago can read zero on a tank at 1 ppm, or the opposite. The first doubt, faced with a result that does not add up, is the date on the bottle.
- Dechlorinators and ammonia binders interfere with some types of test (Nessler-reagent ones in particular): a test run right after a treatment can give a false positive.
When in doubt, take a water sample to an aquarium shop with a different test from your own: two methods that agree are worth far more than one repeated twice.
When zero is not good news
There is one case in which ammonia at zero does not mean what it seems: a new tank where it reads zero because the cycle has not started yet. In week 1, three zeros — ammonia, nitrite, nitrate — mean "sterile water", not "tank ready". It is the misunderstanding that leads to buying fish one week too early, and it clears up in one way only: by checking whether ammonia and nitrite have already appeared and then returned to zero, and whether nitrate is present.
How Gurglee does it
In Gurglee ammonia is one of the basic parameters, with zero as its reference and a warning when it drifts. The useful part, though, is the context: on the parameter's page the app shows the predicted effect of a water change before you do it — how far it would fall with 30% on that tank's volume — so you know in advance whether one change is enough or whether it takes two. And in guided start-up it reads the three cycle values together, telling the "it has not started yet" zero apart from the "it is finished" zero.
Related guides
High nitrite in an aquarium: what to do right now
Nitrite above zero: why it is an emergency, what to do in the next few hours and the next few days, and how long it takes to fall back to zero on its own.
The aquarium nitrogen cycle: what it is and how long it takes
What the nitrogen cycle is, how long it really takes and how to tell when a tank is ready for fish. With the values to check week by week.
New tank syndrome: symptoms and what to do
Fish in a new tank are sick or dying: how to recognise new tank syndrome, what to test straight away and how to limit the damage.
Aquarium pH: ideal values and how to correct it
Which pH to hold for each type of tank, why it swings over the day, and why correcting it directly is almost always a mistake. What to do instead.