Ammonia in a fish tank: new tank syndrome and how to lower it
Symptom: Your ammonia test reads above zero, fish are gasping or lethargic, or the tank is new and fish are dying.
Likely causes, ranked
1. New tank, immature biofilter
very commonDiagnosticTank under two months old, fish added early, ammonia then nitrite rising.
FixUF/IFAS says a new biofilter takes six to eight weeks to build enough bacteria. Feed lightly, change 25–50 % of the water when ammonia shows, and don't add more fish until ammonia and nitrite read zero. Our nitrogen cycle guide walks through it.
2. Overfeeding
very commonDiagnosticFood left on the bottom; ammonia rises after big feeds.
FixUF/IFAS calls overfeeding a major cause of high ammonia: the more fish are fed, the more ammonia they make, and uneaten food adds more. Reduce or stop feeding until levels fall.
3. Biofilter knocked back by medication or cleaning
commonDiagnosticAmmonia appears after treating with medication (especially antibiotics) or replacing all the filter media.
FixUF/IFAS warns that disease treatments, especially antibiotics, can harm nitrifying bacteria; test more often after treating. Rinse media in tank water instead of replacing it all at once.
4. Low oxygen or low alkalinity
less commonDiagnosticPoor surface agitation, or KH near zero.
FixNitrifiers need oxygen and alkalinity; UF/IFAS says they can't function below 20 mg/L alkalinity. Add aeration and restore KH (see the low KH page).
5. Ammonia in the source water
rareDiagnosticFresh tap or well water tests positive before it goes in.
FixUF/IFAS notes city or well water can contain ammonia; test it, and use a water conditioner that binds ammonia for short-term relief while the biofilter handles it.
Common myths
ClaimAmmonia is equally toxic whatever the pH.
RealityUF/IFAS: the un-ionised form is about 100 times more toxic, and there's more of it at high pH and temperature, so the same test reading is more dangerous in hard, warm water.
ClaimAmmonia binders fix the problem.
RealityUF/IFAS says ammonia-binding products give short-term relief only; the long-term fix is a working biofilter.
For a new tank's biofilter
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Live nitrifying bacteria can help seed a new biofilter; they don't replace testing, light feeding and water changes.
Seeds nitrifiers during cycling. Pairs with Ammonium Chloride.
Questions
How do I lower ammonia in my fish tank fast?
UF/IFAS recommends reducing or stopping feeding and a 25–50 % water change; ammonia binders give short-term relief.
What level of ammonia is dangerous?
UF/IFAS says harm starts at about 0.05 mg/L of un-ionised ammonia and fish die at 2.0 mg/L; the un-ionised share rises with pH and temperature, so check both.
What is new tank syndrome?
Ammonia and then nitrite building up in a new tank before the bacteria that process them have grown; UF/IFAS says that takes six to eight weeks.
How long does it take to cycle a fish tank?
UF/IFAS says a new biofilter needs six to eight weeks to build up enough bacteria.
Is nitrite toxic too?
Yes: UF/IFAS says nitrite is toxic at levels as low as 0.10 mg/L. Adding chloride (salt) at 10 mg/L per 1 mg/L of nitrite reduces its toxicity to fish.
Related
Sources & references
- 1.Francis-Floyd, Watson, Petty & Pouder — Ammonia in aquatic systems, UF/IFAS FA16 (2009)https://extension.rwfm.tamu.edu/wp-content/uploads/sites/8/2013/09/Ammonia-in-Aquatic-Systems.pdf
Last reviewed