Water-change regime
Dilution is the primary and most effective response. Large, well-prepared water changes reduce the concentration immediately, and repeated changes hold it down while the biological filter establishes.
Act nowEnvironmentalEnvironmental injury
Ammonia poisoning
Ammonia exists in water as an equilibrium between un-ionised NH₃ and the ammonium ion NH₄⁺. The un-ionised form is uncharged and crosses the gill membrane freely; the ionised form does not, and is far less toxic. The proportion in the un-ionised form rises sharply with pH and, less steeply, with temperature. This is why a bare ammonia number is not a verdict, and why AquariumHQ provides a calculator rather than a threshold. Damage is to the gills first — the tissue in direct contact — producing the respiratory signs that dominate the presentation, and chronic sub-lethal exposure damages gills and kidneys in ways that shorten life long after the reading returns to zero.
Short answer
Ammonia poisoning is chemical injury from ammonia in the water, most often in a tank that is not yet biologically established. The critical point is that a test kit reports total ammonia, but only the un-ionised fraction is seriously toxic — and what fraction that is depends on pH and temperature. The same reading is an emergency in alkaline warm water and a much smaller problem in acidic cool water.
What you can and cannot establish yourself
Suggestive by eye, not conclusive
You can see it, but several conditions look like this. Work through the alternatives below before deciding what you are treating.
- Cause
- Un-ionised ammonia in the water, which crosses the gill membrane freely
- Spread between fish
- Not transmissible
- Urgency
- Act now
- Affects
- fish, shrimp, snail, crayfish, crab, other-invertebrate
- Confidence
- high confidence
Signs
- Gasping at the surface and rapid gill movement
- Reddened or inflamed gills
- Lethargy, sitting on the bottom, or hanging near the filter outflow
- Clamped fins and loss of appetite
- Reddened streaks on the body or fins in more severe exposure
- Loss of coordination or erratic swimming in severe cases
- All or most fish affected at once — the single most useful observation
Do this first: the environmental response
Water quality is the cause, the contributing factor or the deciding variable in the great majority of aquarium health problems. These steps come before any treatment decision, and for several conditions they are the entire treatment.
- Large water change immediately, dechlorinated and temperature-matched. This is the treatment: dilution removes the toxin, and nothing else does.
- Stop feeding until readings return to zero. Uneaten food and the waste from feeding are the source.
- Increase aeration — damaged gills need every advantage, and the fish's oxygen extraction is compromised.
- Do NOT raise pH while ammonia is present. Raising pH converts ammonium into the far more toxic un-ionised form, and doing it to a tank with an ammonia reading can kill fish that were coping.
- Check for a dead fish, a decomposing plant or a stopped filter.
- Test daily until the tank is stable. A single good reading does not mean the filter has recovered.
- Where the tap water is chloraminated, confirm the dechlorinator handles the ammonia released.
What else it could be
Every entry names the observation that separates it from this record. Work through these before deciding what you are treating — the most expensive mistake in fish health is confident treatment of the wrong thing.
Non-disease explanations — check these first
How to tell it apart: Produces very similar respiratory signs and occurs in the same circumstances, usually a few weeks later in a cycling tank. Test both — they are frequently present together and the response differs in one important respect.
How to tell it apart: Also affects all fish at once with the same gasping. Distinguished by testing ammonia, and by the fact that hypoxia improves within minutes of vigorous aeration while ammonia injury does not.
How to tell it apart: Onset immediately after a water change, with all fish affected within minutes rather than over hours. Ask what went into the tank and when.
Other conditions
How to tell it apart: Parasitic gill disease usually affects individuals unevenly and progresses over days, and the water tests clean. A clean test is what moves the investigation on.
How to tell it apart: Look for the fine gold dusting under a torch. Water tests clean.
What makes it more likely
Each entry states the mechanism, not just the association — an association without a mechanism is not something you can act on.
- High pH. The proportion of total ammonia present in the toxic un-ionised form rises steeply with pH. At pH 8.5 a reading is far more dangerous than the same reading at pH 6.5.
- High temperature. Raises the un-ionised fraction, and simultaneously reduces dissolved oxygen and raises the fish's oxygen demand at the moment its gills are damaged.
- A new or recently disturbed filter. Nitrifying bacteria are slow-growing and are the whole of the tank's ammonia-processing capacity. Anything that removes them removes it.
- Chloramine in the water supply. A dechlorinator that breaks the chlorine-ammonia bond releases ammonia into the tank. Where the supply is chloraminated this needs a product that handles the ammonia as well.
Treatment categories
AquariumHQ names classes of response and never doses. Product concentrations differ between markets and formulations, several actives are temperature- and pH-dependent, and the invertebrate and plant tolerances that decide whether the rest of the tank survives are not printed on the label. Where a category puts a chemical in the water, what it can damage is stated with it.
Increase aeration
Compensates partly for compromised gill function and supports the nitrifying bacteria, which are aerobic and whose activity is oxygen-limited.
Correct the husbandry
Reducing feeding, reducing stocking and allowing the biological filter to establish addresses the cause rather than the reading.
Veterinary referral
Rarely required for the exposure itself, which is a water problem. Appropriate where fish are severely damaged and welfare decisions arise, or where secondary infection follows the gill and skin damage.
Isolation and quarantine
- Isolation is meaningless here unless the destination water is better. The problem is the water, and every fish in it is affected.
- Moving fish into a bucket of untreated tap water, or into a second uncycled tank, makes things worse. If fish must be moved, move them into water of known quality and matched temperature.
When a vet is the right call
- Fish severely damaged, with welfare decisions to make
- Secondary infection developing in gill or skin damage after the water has been corrected
- Continued losses after ammonia has returned to zero, which indicates something else is also wrong
AquariumHQ describes what is commonly reported and what husbandry correction is usually indicated. It does not diagnose and is not a substitute for an aquatic veterinarian. Many treatments are given to healthy fish suffering from a water-quality problem, which makes the outcome worse rather than better.
Prevention
- Cycle a tank before adding fish, and understand that the process takes weeks rather than days.
- Add stock gradually rather than all at once.
- Never clean filter media in untreated tap water — the chlorine kills the bacteria you are relying on.
- Use a dechlorinator appropriate to your water supply, and check whether it is chloraminated.
- Test after any change: new fish, a filter service, a medication course, a power cut.
- Do not overfeed.
What AquariumHQ does not know
Published rather than hidden. A health page that lists no limitations has not had its limitations looked for.
- Species-level sensitivity differences are documented in the aquaculture literature for food species and are largely unknown for ornamental species. AquariumHQ does not publish per-species thresholds it cannot support.
- The long-term consequences of repeated sub-lethal exposure are described qualitatively in the literature and not quantified in a form useful to a keeper.
Evidence
Each claim is shown with what kind of support stands behind it and how strongly we hold it. Those are separate axes: a measurement held tentatively and an editorial position held firmly are different things.
Observed / measuredhigh confidence
Un-ionised ammonia (NH₃) is the toxic form; the ammonium ion (NH₄⁺) is substantially less toxic, and the equilibrium between them is governed by pH and temperature
- Emerson et al. (1975), Aqueous ammonia equilibrium calculations: effect of pH and temperature — Aqueous ammonia equilibrium — effect of pH and temperature
- US EPA aquatic life ambient water quality criteria for ammonia (freshwater) — Aquatic life ambient water quality criteria for ammonia — freshwater
Last verified 2026-08-11
Derived from other datahigh confidence
Raising pH in the presence of total ammonia increases the un-ionised fraction and therefore the toxicity, so pH must not be raised while ammonia is present
Derived from the stored equilibrium relation rather than quoted; the calculation is exposed in the free-ammonia tool.
- Emerson et al. (1975), Aqueous ammonia equilibrium calculations: effect of pH and temperature — Aqueous ammonia equilibrium
- AquariumHQ husbandry synthesis — AquariumHQ derivation from the ammonia equilibrium relation
Last verified 2026-08-11
Sourced recommendationhigh confidence
The gill is the principal target tissue, producing respiratory signs as the dominant presentation
- MSD/Merck Veterinary Manual — fish chapters — Fish — ammonia toxicity
- UF/IFAS Extension — fish health and aquaculture publications — Water quality — ammonia in aquaculture and aquarium systems
Last verified 2026-08-11
Sourced recommendationhigh confidence
The effective response is water exchange and cessation of input; no medication addresses ammonia exposure
- UF/IFAS Extension — fish health and aquaculture publications — Water quality management
- MSD/Merck Veterinary Manual — fish chapters — Fish — water quality disorders
Last verified 2026-08-11