0.25 ppm ammonia is always dangerous.

It depends on pH and temperature. At pH 6.5 and 24 °C the free-ammonia fraction is very small; at pH 8.5 the same reading is serious. The reading alone is not a verdict.

Ammonia comes only from fish.

Decaying food, dead plant matter, a dead fish behind the filter and disturbed anaerobic substrate are all common sources — often the source when a stable tank suddenly reads positive.

compoundNH₃mg/L

Ammonia

A hobby test kit reports TOTAL ammonia nitrogen — free ammonia (NH₃) plus ammonium (NH₄⁺) combined. Only NH₃ passes readily across the gill membrane, and the proportion present as NH₃ rises steeply with pH and with temperature. The same 1 mg/L reading is a minor problem at pH 6.5 and an emergency at pH 8.5.

In one sentence

Ammonia is the primary nitrogenous waste of fish and the most acutely toxic compound in a freshwater aquarium. How dangerous a given test reading is depends on pH and temperature, because only the un-ionised NH₃ fraction is highly toxic.

How to read a measurement

Bands are guidance for interpreting a reading, not thresholds with sharp edges. Sensitivity varies by species and by how long the exposure lasts.

Band	Range	Verdict	What it means
Not detected	≤ 0.02	target	Normal for a cycled, correctly stocked tank.
Trace	0.02–0.25	watch	Something has changed. Check stocking, feeding and the filter.
Detectable	0.25–1	act	Water change now, and stop feeding until it clears.
High	≥ 1	urgent	Immediate large water change. At alkaline pH this range causes gill damage within hours.

Why pH changes everything

Ammonia in water sits in equilibrium with ammonium: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. At low pH the equilibrium sits far towards ammonium, which is comparatively harmless. At high pH it shifts towards free ammonia, which is not.

The dissociation constant is temperature-dependent, so warmer water also increases the toxic fraction. AquariumHQ's free-ammonia calculator applies the relation published by Emerson and colleagues in 1975, which is the one used throughout water-quality practice.

Approximate proportion of total ammonia present as toxic free NH₃, fresh water at 25 °C
pH	Free NH₃ share
6.5	about 0.2 %
7.0	about 0.6 %
7.5	about 1.8 %
8.0	about 5.4 %
8.5	about 15 %
9.0	about 36 %

The water-conditioner trap

Several dechlorinators claim to 'detoxify' ammonia. What they generally do is bind ammonia into a less toxic complex that most test kits still read as ammonia. The reading therefore stays high after treatment even though the immediate danger is reduced, and the product does not remove the nitrogen from the system.

This matters practically: after using such a product, an ammonia test is no longer a reliable measure of danger, and a Nessler-type kit may read differently from a salicylate-type kit on the same sample.

What to do about a reading

Change water. A 50 % change halves the concentration and is the fastest lever available.
Stop feeding for a day or two. Uneaten food and reduced excretion both matter.
Check the filter is running and has not been over-cleaned.
Do NOT raise the pH while ammonia is present — that converts ammonium to free ammonia and makes things acutely worse.
Increase aeration. Ammonia damages gills, and a fish with damaged gills needs all the dissolved oxygen it can get.

How it is measured

Liquid test kit (salicylate or Nessler method)
Test strips (less reliable)
Continuous ammonia alert badges

Typical aquarium range

Typical: 0–0 mg/L

Common misconceptions

Often said	What is actually the case
0.25 ppm ammonia is always dangerous.	It depends on pH and temperature. At pH 6.5 and 24 °C the free-ammonia fraction is very small; at pH 8.5 the same reading is serious. The reading alone is not a verdict.
Ammonia comes only from fish.	Decaying food, dead plant matter, a dead fish behind the filter and disturbed anaerobic substrate are all common sources — often the source when a stable tank suddenly reads positive.