When fish suddenly gasp at the surface, refuse food or become less active, first test ammonia, nitrite, pH and water temperature, and check dissolved oxygen and equipment. Water quality poisoning can look like an infection and can also make fish more prone to secondary infections; the two can coexist.[1][2]
What to do first when you detect ammonia or nitrite
First reduce or pause feeding, remove leftover food and dead organic matter, and make sure filtration and aeration are working. A brief pause in feeding usually has limited effect on most healthy adult fish; fry and other fish that need special feeding arrangements shouldn't be handled the same way.[1]
Water changes can reduce pollutants in small systems, but the new water must be properly treated, and its temperature, pH and source water quality checked. Keep retesting after the water change until the system can reliably handle its original load. If fish are still breathing rapidly, losing balance or dying one after another, contact a veterinarian at the same time.[1][2]
Be especially careful with systems that have very high total ammonia and are on the acidic side (the Merck Veterinary Manual gives total ammonia above 5 mg/L and pH below 7 as an example). If a large water change suddenly raises the pH, the larger share of ammonium ions may convert into the more toxic un-ionized ammonia. In this extreme situation, the Merck Veterinary Manual recommends moving the fish to a clean system already adjusted for pH and temperature; the actual approach needs to be assessed by a veterinarian or other professional.[2]
Symptoms of ammonia and nitrite poisoning
| Item | Ammonia | Nitrite |
|---|---|---|
| Common signs | Lethargy, refusing food, darkened color; in acute cases, possible spinning or convulsions. | Gasping at the surface and rapid breathing; fish may look oxygen-starved even when there's oxygen in the water. |
| Main damage | Damages the gills and other tissues and disrupts physiological functions. | Oxidizes hemoglobin to methemoglobin, reducing oxygen-carrying capacity. |
| Visible clues | Usually no sign that confirms it on its own. | Blood or gills may look brown, but not seeing this doesn't mean there's no poisoning. |
Symptoms vary with species, concentration and exposure time, and the table can only suggest directions. Gasping at the surface may also come from low dissolved oxygen or gill infections.[2][3]
Why the same ammonia reading can carry different levels of danger
Total ammonia in water includes un-ionized ammonia NH₃ and ammonium ions NH₄⁺, and the former is more toxic. The higher the pH and water temperature, the larger the share of un-ionized ammonia generally is; calculations for saltwater also need to account for salinity. Total ammonia readings should be interpreted together with these conditions.[1]
| Freshwater conditions | pH 7 | pH 8 | pH 9 |
|---|---|---|---|
| 20°C | approx. 0.39% | approx. 3.81% | approx. 28.4% |
| 30°C | approx. 0.80% | approx. 7.43% | approx. 44.5% |
The table shows the proportion of total ammonia in un-ionized form, based on the Emerson equilibrium data listed by UF/IFAS, and can't be used as a lethal threshold.[1] Within the low range, each one-unit rise in pH can increase the proportion nearly tenfold; at higher pH, the fixed tenfold rule no longer applies.
A stable aquarium system should aim for undetectable ammonia and nitrite. A detectable reading means the load and filtration need checking, but it doesn't mean every reading is immediately lethal; the actual risk depends on the species, water conditions and exposure time.[1][2]
Check the test kit's units before calculating
TAN stands for total ammonia nitrogen. If it's expressed in mg N/L, multiplying it by the un-ionized ammonia fraction still gives a result expressed as nitrogen, NH₃-N. To convert to NH₃ by molecular weight, multiply by about 17/14. If a test kit label uses a different notation, convert according to its instructions.[4]
For example, with TAN at 1 mg N/L, a water temperature of 30°C and pH 8, using a fraction of 0.0743: un-ionized ammonia is about 0.074 mg NH₃-N/L; converted by molecular weight, it's about 0.090 mg NH₃/L. This is a calculation example, not a determination of a safe concentration.
| Label | Meaning | Conversion if needed |
|---|---|---|
| mg N/L or TAN as N | Total ammonia expressed as weight of nitrogen | Multiply by the fraction to get NH₃-N; then multiply by about 1.21 to get NH₃. |
| NH₃-N | Un-ionized ammonia expressed as weight of nitrogen | Multiply by about 1.21 to get NH₃. |
| NO₂-N | Nitrite expressed as weight of nitrogen | Multiply by about 3.29 (based on molecular weights, 46/14) to get NO₂⁻. |
| NO₂⁻ | Nitrite expressed by molecular weight | Divide by about 3.29 (based on molecular weights) to get NO₂-N. |
Before comparing literature or product data, make sure the concentrations, basis of measurement and water conditions match. Don't compare values directly just because they're both in mg/L.[2][4]
Some water treatment products and test methods can interfere with each other. UF/IFAS specifically warns that the Nessler method may be affected by some water treatment products or certain medications; if you get unusual readings after using a water treatment product, check the test method and compatibility rather than adding more treatment based on a single result.[1]
Why nitrite can make fish in oxygenated water look oxygen-starved
Nitrite enters the fish through the gills and reduces the blood's ability to carry oxygen. Chloride ions can compete with nitrite for some uptake pathways, so tolerance varies widely among fish species and different bodies of water; chloride ions are not the same thing as the free chlorine used for disinfection.[3]
This mechanism can't be turned directly into a treatment that works for every species. Fish species, water quality and other organisms differ in tolerance; no additive can replace load control, water changes and biological filtration. Whether additional treatment is needed should be assessed by a veterinarian based on actual water quality.
Judge new tank syndrome by load and monitoring
When new filter media haven't yet built up enough nitrifying capacity, ammonia may rise first, followed by nitrite buildup. The literature often gives several weeks for establishment, for example six to eight weeks, but the time depends on the seeding source, oxygen, alkalinity and load. A tank's age can't replace water quality monitoring.[1][2]
Nitrifying microbes aren't limited to the two bacteria in common textbooks. A 2024 study detected clade A comammox Nitrospira in all 38 freshwater aquarium filter samples; in 30 of them, the related ammonia-oxidation functional genes were dominant. This supports its importance in many freshwater filters, but functional gene counts aren't a direct measurement of nitrification rates, nor can they be extrapolated to mean the same group of microbes dominates every aquarium.[5]
Whatever the nitrifying microbes, what owners need to maintain is enough filter media, steady aeration and an appropriate load. Some medications, very low oxygen or insufficient alkalinity can all reduce the system's capacity; increase monitoring after medicating and after adding fish.[1][2]
Frequently asked questions
Once a new tank has been running for eight weeks, can I add lots of fish at once?
You can't go by time alone. Newly added fish and feeding increase the load, so add gradually and track ammonia and nitrite to make sure the system can handle it.[1]
Water quality is normal, but my fish is still gasping. What should I do?
Check for gill disease, other toxins or damage from earlier exposure, and also make sure the test kit is still good and the measurements are accurate. For veterinary care and sample submission, see "What to Do When Your Fish Is Sick".
After cutting back on feeding, do I still need to fix the filtration problem?
Yes. Feeding less temporarily reduces the load, but in the long run you still need to fix the system's capacity and stocking density; don't keep fish hungry for long periods just to keep readings looking normal.[1]
If medication is needed, use a legally registered veterinary drug that's appropriate for the case, following the label and your veterinarian's instructions.
Further reading
- What to Do When Your Fish Is Sick – Reading Aquarium Fish Symptoms and What to Do First
- How to Quarantine New Fish – Isolating for at Least 30 Days and Acclimating Fish to the Tank
- Is White Fuzz on Your Fish Water Mold? – Wound Infections and Why Fish Eggs Grow Fuzz
References
- Francis-Floyd R, Watson C, Petty D, Pouder DB. Ammonia in Aquatic Systems. UF/IFAS, FA031, 1990 (revised 2022).
- Francis-Floyd R, Yanong RPE, Petty BD. Environmental Diseases of Aquatic Animals in Aquatic Systems. Merck Veterinary Manual, revised 2023.
- Durborow RM, Crosby DM, Brunson MW. Nitrite in Fish Ponds. SRAC Publication 462, 1997.
- Florida Department of Environmental Protection. Explanation of the Discrepancy between the Florida Department of Environmental Protection's Un-ionized Ammonia Calculator (Version 2.1) and the EPA Text Table 3 Values in EPA 440/5-88-004, accessed October 2026.
- McKnight MM, Neufeld JD. Comammox Nitrospira among dominant ammonia oxidizers within aquarium biofilter microbial communities. Applied and Environmental Microbiology, 2024;90(7):e00104-24. doi:10.1128/aem.00104-24.