Ammonia in an aquarium: why the same reading is harmless at pH 6.5 and lethal at pH 8
Published by Planète Pets · managing editor: Clément Lacaille · how our guides are written
A test kit turning pale green a few days into a new setup worries every beginner, but the number on the chart only tells part of the story. The reference calculations by Emerson, Russo, Lund and Thurston, published in 1975 in the Journal of the Fisheries Research Board of Canada, established the exact proportion of genuinely toxic ammonia present in water depending on pH and temperature (see the study). Our maintenance and water-quality guides are gathered in the fish care and grooming section.
Contents · 4 sections
A test measures two molecules, only one is dangerous
What off-the-shelf kits measure is total ammonia nitrogen: the sum of free ammonia (NH3) and the ammonium ion (NH4+). Both forms coexist in the water and convert into one another depending on conditions. Only free ammonia crosses gill membranes easily, though: the ammonium ion carries an electrical charge and is far harder to take up. Two tanks showing the same 0.5 mg/L can therefore be in radically different situations.
pH decides, temperature fine-tunes
The split between the two forms follows a very steep curve. At 25 °C, roughly 0.6% of total ammonia nitrogen sits as free ammonia at pH 7; that fraction rises to about 5.4% at pH 8 and climbs to roughly 36% at pH 9. Each extra pH unit therefore multiplies the toxic share by close to ten. Temperature pushes in the same direction but more quietly: at pH 8, the free-ammonia fraction runs around 3.8% at 20 °C versus 5.4% at 25 °C. A cichlid tank at pH 8.2 and 26 °C and an Amazon-style tank at pH 6.5 and 24 °C are nowhere near the same risk for an identical reading — which is why knowing your values matters, as our guide to GH, KH and pH in aquarium water explains.
What ammonia actually does to a fish
The review by Randall and Tsui, published in 2002 in Marine Pollution Bulletin, describes the mechanism (see the study): excess ammonium displaces potassium and depolarises neurons, which activates NMDA-type glutamate receptors, triggers a massive influx of calcium and ends up killing cells in the central nervous system. The authors also point out that most species excrete their own ammonia by diffusion across the gills: when the water already contains some, that route slows down and the fish is poisoned by its own metabolism. Our articles on nitrite and haemoglobin and on nitrogen-cycle water tests round out the picture.
- About 0.6% free ammonia at pH 7 and 25 °C, versus 5.4% at pH 8 and 36% at pH 9.
- Close to a tenfold rise per extra pH unit: by far the most decisive parameter.
- 3.8% versus 5.4% at pH 8 depending on whether the water sits at 20 °C or 25 °C.
- Gill excretion blocked when the water is already loaded: the fish can no longer offload its own ammonia.
In practice, an ammonia reading is never read on its own: it is read alongside the pH and temperature logged the same day. It is also why the pH of a loaded tank should never be pushed up abruptly — the same amount of nitrogen would suddenly become far more aggressive.
Frequently asked questions
My test shows 0.25 mg/L of ammonia, should I worry?
At pH 6.5 and 24 °C, the genuinely toxic fraction stays very low; at pH 8.2 and 27 °C, the same value warrants an immediate water change and a pause in feeding.
Should I lower the pH to protect my fish during a spike?
No: an abrupt pH swing is itself a major stressor. Dilute instead with partial water changes at matching parameters and let the filter finish its work.
Why does warm water make the problem worse?
It shifts the equilibrium slightly towards free ammonia, while speeding up fish metabolism so they produce more of it, and reducing the dissolved oxygen available.
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This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.
