What Causes Ammonia in Fish Tank Water, and What It Means

The short answer

Fish produce ammonia as waste, uneaten food and decay add more, and filter bacteria remove it. A reading above zero usually means waste is outpacing those bacteria: a new tank cycling, extra fish or food, something dead, a damaged or struggling filter, or ammonia that came in with the tap water. A test can also mislead (see the kit notes).

The University of Florida’s IFAS Extension, a university extension service, notes that “ammonia is colorless and, even at levels toxic to fish, odorless”, so only a test shows it. The useful question is what changed: the tank’s age, the stocking, the feeding, the last filter clean or the last water change.

Warning

If fish look unwell, gasp at the surface or stop eating, test ammonia, nitrite, pH and temperature now and contact an aquatic veterinarian. IFAS says water should be tested immediately “any time fish appearance or behavior changes (including reduced feeding)”. This article covers the water only and gives no fish treatment or medication advice.

Where aquarium ammonia comes from

IFAS defines it plainly: “In fish, this primary metabolic waste product is ammonia.” Aqueon, an aquarium equipment and water-care maker, explains: “Ammonia is expelled through the fish’s gills, as well as eliminated through urine and feces. It can also build up in your aquarium when uneaten food and other organic materials decompose.” Its nitrogen cycle guide adds dead plant material. IFAS says that in most hobbyist systems the feed the fish eat and digest is the primary source: “The more feed a fish is fed, the more ammonia the fish will produce. However, even a starved fish will produce some ammonia.”

Removal is the bacteria’s job. IFAS explains that bacteria such as Nitrosomonas convert ammonia to nitrite, and a second group converts nitrite to “the much less toxic nitrate”. Aqueon says these bacteria “are naturally present in your water, but you need a filter so you can grow enough bacteria to maintain a healthy nitrogen cycle.” So in a running tank, the reading usually reflects the gap between the waste going in and what the beneficial bacteria take out. Nitrite is the next link in the chain; see our guide to why nitrite gets high in an aquarium.

Causes in a new tank

Aqueon’s freshwater water-quality guide says ammonia “tends to rise in newly established aquaria due to a lack of nitrifying bacteria, adding too many fish at once, overfeeding or a combination of these factors”, and warns that “simply allowing your aquarium to run for 24 to 48 hours after first setting it up is not cycling.”

Timing: Aqueon, writing for freshwater tanks, says “ammonia levels typically begin to rise by the third day after you’ve added fish to your new aquarium”, and the whole cycle “typically takes three to eight weeks.” IFAS, writing for tanks and aquaria, says a new biofilter needs “six to eight weeks to build up sufficient bacteria to effectively reduce ammonia and nitrite levels.”
Cleaning too early: Aqueon, writing about a newly set-up aquarium, warns against “cleaning the filter when it doesn’t really need it, as this can destroy beneficial bacteria.”
Why a big spike drags on: Seachem, a water-treatment and test-kit maker, writes that “free ammonia is toxic to nitrifying bacteria and that high ammonia concentration is a major cause of difficulty in establishing a biological filter.”

Aqueon counts the tank as cycled when “nitrates are being produced, and ammonia and nitrite levels are at zero.” If weeks pass without that, our guide on why an aquarium won’t cycle covers the reasons a cycle stalls.

Causes in an established tank

IFAS: “In healthy ponds and tanks, ammonia levels should always be zero. Presence of ammonia is an indication that the system is out of balance.” Either more waste is going in, or the bacteria are removing less. The clues below are prompts for where to look, not diagnoses.

More fish than the filter can handle

Aqueon says ammonia “can also rise in heavily populated tanks due to generous feeding and/or insufficient filtration”, and API, a test-kit maker, names “populating a small aquarium with too many fish for the amount of water” in its FAQ. Clue: new fish went in recently, or the fish have outgrown the tank.

More food than the fish eat

IFAS calls overfeeding “a major cause of high ammonia concentrations”, and API’s FAQ says uneaten food “spikes ammonia levels as it decays”. Clue: food left on the substrate after feeding.

Something dead or rotting

Answering a question about a smelly tank, API’s FAQ points to food waste “or even a dead fish or snail that went unnoticed”. Aqueon notes that ammonia levels increase as plants break down, and API that buried waste matters: “Even small amounts of waste can spike ammonia and nitrite levels in your tank.” Clue: a head count comes up short, or plants are decaying. For tracking down a bad smell, see why a fish tank smells.

The filter lost its bacteria

Aqueon lists “a biofilter that’s too dirty or too clean” as a cause. Practical Fishkeeping, a fishkeeping magazine, says new media and “media which has been washed in tapwater or replaced with new” hold few bacteria; its rule is to “only wash filter media in old tank water, not under the tap”. Clue: ammonia rose within days of a filter clean or a new cartridge. See our guide to cleaning a hang-on-back filter with tank water.

The bacteria are struggling

IFAS names three conditions. Oxygen: “If oxygen levels are not sufficient, the bacteria can stop functioning or die, and ammonia and nitrite levels will increase.” Alkalinity, which aquarium kits sell as KH (Aqueon’s term is “KH (Carbonate Hardness or Alkalinity)”): “At a total alkalinity less than 20 milligrams per liter (mg/L), common species of nitrifying bacteria do not function effectively.” Medication: “Some chemicals used to treat diseases in fish, especially antibiotics, can be detrimental to the nitrifying bacteria in the biofilter.” Aqueon adds heat: “above 90°F, the beneficial bacteria that maintain balance in an aquarium begin to die.” Clue: the reading rose after a heat wave or a treatment dosed into the tank, or a KH test comes in under IFAS’s figure (compare in mg/L). Medication itself is a question for an aquatic veterinarian.

Filtration too small, or none

Aqueon’s advice is to “check that your biofilter is large enough for your tank.” Clue: a bowl with no filter, or a filter rated for a smaller tank.

Ammonia that shows up after a water change

A water change should dilute ammonia. When the reading rises instead, look at the new water and at the test.

The tap water. Two different routes, from two sources. IFAS notes that “ammonia may be present in source waters such as some municipal (city) or well waters.” Separately, the EPA says “more than one in five Americans uses drinking water treated with chloramines”, which “are most commonly formed when ammonia is added to chlorine to treat drinking water”, and Seachem writes that with chloramine, thiosulfate “has proven inadequate, causing the release of ammonia”; it adds that this is not serious at acid or neutral pH, but “at alkaline pH it can be devastating”. The EPA says chlorine and monochloramine “must be neutralized or removed if used in aquariums”, and that “water utilities typically provide consumers with information about disinfection processes”, so your utility can tell you whether it uses chloramine; check that your conditioner’s label names it. Questions about your drinking water go to your water supplier or health department.
The conditioner and the kit. Ammonia binders change what some kits show, and the sources disagree on which. IFAS says binders and many water conditioners “will also cause false high ammonia readings with the Nessler’s method”, while the salicylate method “is not affected by ammonia binding products or formalin treatments.” Seachem, which sells its own free-ammonia kit, says salicylate and Nessler kits raise the sample’s pH so that “all ammonia removal products will breakdown and rerelease the ammonia, thus giving you a false ammonia reading.” Either way, API says its Ammo-Lock “does not remove ammonia from water; it converts it to a non-toxic form”, so the bacteria still have to remove it.
Gravel cleaning. Seachem warns of “the ammonia and nitrite spikes that can result from over-enthusiastic gravel cleaning.”
An old kit. Aqueon’s reminder: “Use current water testing kits (they can expire!)”.

What the number on your kit means

Aqueon notes that “aquarium water test kits usually measure NH₃ and NH₄+ combined”, and IFAS that kits read in mg/L, “which is the same as parts per million (ppm).” IFAS says the un-ionized form (NH₃, which Seachem calls free ammonia) is “approximately 100 times more toxic to fish than the ionized (NH4+) form”, and its share “increases with increasing pH and/or temperature but decreases slightly with increasing salinity.”

Seachem gives numbers: at pH 8.3, free ammonia “makes up about 10% of the total”, and at pH 7.0 “less than 0.5% of the total.” Worked from those two figures, the toxic share of the same kit reading is roughly 20 times larger or more at pH 8.3 than at pH 7.0 (Seachem gives no temperature for either figure). So IFAS says “a good ammonia test kit will include a TAN test, a pH test, and a thermometer”; our guide to what causes high pH in an aquarium covers the pH side.

Examples, in the makers’ words: the API Ammonia Test Kit, a liquid kit, “reads ammonia levels from 0 to 8 ppm”; API’s ammonia test strips read “0-6 ppm (mg/L)”; Seachem’s MultiTest Ammonia “measures total (NH3 and NH4+) and free ammonia (NH3 only).”

The target is zero: Aqueon says “the only safe ammonia level is zero.” Read ammonia next to nitrite and nitrate:

What the kits show	Usually points to	Check next
New tank: ammonia up, nitrite zero	Cycle has started	Test every 2-3 days (Aqueon)
New tank: ammonia falling, nitrite up	Cycle moving on to nitrite	Keep testing both
Ammonia and nitrite zero, nitrate present	Cycled (Aqueon)	Test weekly
Established tank: any ammonia	Waste outran the bacteria	Causes 1-6 above
Ammonia in untreated tap water	Source water can carry ammonia (IFAS)	Ask your water utility
Ammonia after a thiosulfate-only conditioner on chloraminated water	It can release ammonia (Seachem)	Ask your water utility whether it uses chloramine; check the conditioner maker’s label
High right after a binder	Possibly the kit	Kit maker’s instructions

While you look for the cause

Two first moves come from IFAS. Test nitrite, pH and temperature as well as ammonia, since IFAS says pH and temperature must be known to work out how much of the reading is the toxic form. And cut the feeding: “The first thing to do when ammonia is present in a pond or tank is to reduce or stop feeding.” A short stop of 1-3 days, IFAS says, “will not typically have a negative effect on fish (except fry)”. Water changes, conditioners and how often to retest are covered in our step-by-step guide to lowering ammonia.

Ready to buy?

Our roundup of freshwater aquarium test kits compares test kits and test strips, and our Water Care & Testing section gathers the test kits and conditioners.

Frequently Asked Questions

Q. Can fish recover from high ammonia?

That depends on the fish, the level and the exposure, which is a question for an aquatic veterinarian. IFAS notes that “even after an ammonia problem has been corrected, fish may become sick weeks later.”

Q. Is 0.25 ppm ammonia a problem?

By the sources’ rule, yes: Aqueon says “anything higher than zero is a problem!” How harmful a given reading is depends on pH and temperature.

Q. How long does it take for ammonia to go away?

In a new tank, until the bacteria catch up (the timings above). For an established tank the sources give no timeline; IFAS says “whenever ammonia is found, increase the frequency of testing until the problem is corrected.”

Q. Do plants help get rid of ammonia?

Aqueon says live plants help by “absorbing ammonia and nitrates”, but decaying plants add ammonia.

Sources (17)

Aquarium Tales Team

Written and reviewed by the Aquarium Tales Team. See the About page for how we work.