Ammonia in fish tank: what to do about an ammonia spike

In a healthy, cycled tank, ammonia should read zero, so any reading above zero means something is wrong. Do a 50% water change with dechlorinated, temperature-matched water now, stop feeding, and find the source.

Ammonia is the main waste product fish make, mostly through their gills. Uneaten food, dead fish, rotting plants and poop break down into it too (UF/IFAS). In a healthy tank, bacteria in your filter turn ammonia into nitrite, then into nitrate, which is much less harmful. This is the nitrogen cycle, and a tank where those bacteria are established is called "cycled" (see how to cycle a fish tank). A spike happens when ammonia is made faster than the bacteria can handle it.

How to identify it

Fish less active, hanging near the surface or the filter outlet, less interested in food.
Cloudy water and a bad smell (see cloudy water).
Gasping at the surface, fast gill movement, red or inflamed gills.
Darkened colour, lying on the bottom, spinning or convulsive swimming (Merck Veterinary Manual).

Easily confused with: nitrite poisoning (it causes brown gills and gasping, so test for it too), low oxygen, chlorine, overheating, and gill parasites like flukes. A test kit sorts this out in five minutes.

When it's a problem

How dangerous a reading is depends on pH and temperature. Your test kit measures "total ammonia", which is made of two forms:

Free ammonia (NH3): the toxic form. It passes easily into fish gills and burns them.
Ammonium (NH4+): the ionized (electrically charged) form. UF/IFAS says NH3 is roughly 100 times more toxic.

The higher the pH and the warmer the water, the more of your ammonia is the toxic NH3. Using the UF/IFAS table:

pH	24°C (75°F)	28°C (82°F)
7.0	about 0.5% is toxic NH3	about 0.7%
8.0	about 5%	about 6.5%

So 1 mg/L (1 ppm) total ammonia is about ten times more dangerous at pH 8 than at pH 7. UF/IFAS says tissue damage begins above about 0.05 mg/L of free NH3. A "small" reading in a hard, alkaline, warm tank (African cichlids, livebearers) can be a crisis, while the same number in a soft, acidic tank may be survivable for a while.

Early: a reading above zero, fish a little sluggish or off their food, cloudy or smelly water.
Serious: gasping, fast gills, red gills, especially in warm or high-pH water.
Emergency: fish lying on the bottom, spinning, or several dying within hours or a day.
Normal, don't worry: a steady zero reading in a cycled tank. Anything above zero is worth acting on.

What to do right now

Test ammonia, nitrite, pH and temperature. A liquid test kit is more reliable than strips.
Do a 50% water change with dechlorinated water matched to the tank's temperature. The Merck manual advises at least 50% when free ammonia is high, and stopping or reducing feeding. Exception: if your pH is very low (below about 6.5) and the tank hasn't had a water change in months, read the pH crash guide first. Raising pH fast can turn harmless ammonium into toxic ammonia.
Stop feeding for 1–3 days. Fish can safely go that long; food is fuel for more ammonia.
Find and remove the source: a dead fish (check caves, behind rocks and filters), rotting food, melting plants.
Add an ammonia-binding conditioner. It buys time (roughly a day) while you do water changes; it doesn't remove the ammonia.
Add aeration. Gill-damaged fish need oxygen, and so do your filter bacteria.
Test again in 12–24 hours and repeat water changes until ammonia stays at zero.

What to do next

Water changes are the treatment

There's no medicine for ammonia. You dilute it with clean water until the bacteria catch up. A 50% change halves the ammonia; two 50% changes a few hours apart bring it to a quarter. Always use a dechlorinator that handles chloramine, and match temperature within 1–2°C (2–4°F).

Lower the toxic fraction, carefully

You can't safely drop pH fast in an emergency, so don't push pH down with chemicals. But don't make things worse: don't add pH-raising buffers while ammonia is present, and don't let the tank overheat. A degree or two cooler (within your fishes' range) lowers both the toxic fraction and the fish's oxygen needs.

Help the filter recover

Don't clean or replace the filter during a spike. The bacteria you need live in it.
Bottled bacteria products may help a struggling filter. Results vary; they're not a substitute for water changes.
Zeolite (ammonia-absorbing stones) can help in freshwater, but it fills up and must be replaced.
Fast-growing and floating plants take up ammonia directly.

Cautions

Scaleless fish, plecos, loaches and shrimp are often among the first to suffer. Shrimp are very sensitive.
Many medicines and some antibiotics harm filter bacteria and can trigger a spike. Test daily during any treatment.
Gills damaged by ammonia take weeks to heal. Fish may get fin rot or other infections in the days after a spike, so keep the water clean.

How long it takes

In a cycled tank with one cause (say, a dead fish), ammonia can be back to zero in 1–3 days. In an uncycled tank, expect weeks of careful management. The Merck manual's pet owner page says new tank syndrome usually needs intensive management for about 1–2 weeks. Keep testing daily until you get several zero readings in a row.

Products that help

Liquid ammonia test kit: more reliable than strips. Use it whenever fish act strangely, and daily during a spike.
Nitrite and pH test kits: rule out nitrite poisoning and tell you how toxic your ammonia reading is.
Water conditioner that handles chloramine and binds ammonia: makes new water safe and holds ammonia in a less toxic form for about a day while you change water.
Air stone and air pump: extra oxygen for damaged gills and for the filter bacteria.
Bottled filter bacteria: may help a struggling filter catch up. Results vary.
Zeolite: absorbs ammonia in freshwater until it fills up; then replace it.
Fast-growing or floating plants: take up ammonia directly, every day.
Continuous ammonia alert or monitor: useful if you have many tanks.

Before trouble starts, keep a liquid test kit, a spare bottle of ammonia-binding conditioner and an air stone on hand.

How to prevent it

Cycle new tanks before adding fish. It takes weeks, usually around 6–8 according to UF/IFAS. See how to cycle a fish tank.
Add fish a few at a time.
Feed only what is eaten in a couple of minutes, and remove leftovers.
Rinse filter media in old tank water, never under the tap, and never replace all the media at once.
Test ammonia weekly, and any time fish act strangely.
Count your fish daily, so a missing one gets found before it rots.

Why it happened

Uncycled tank: fish added before the filter bacteria grew. This "new tank syndrome" usually shows up 1–3 weeks after setup (Merck).
Overfeeding, especially heavy foods left to rot (see overfeeding).
A dead fish or snail rotting out of sight.
Over-cleaning the filter: replacing all the media, or rinsing it under tap water.
Medicines that harm filter bacteria.
Too many fish added at once, or overstocking.
A power failure that starved the filter of oxygen.
Chloramine tap water, which releases some ammonia when it's neutralised.

When to get help

If ammonia won't come down after several large water changes, if your tap water itself contains ammonia (test it), or fish keep dying, ask a good local fish store or a vet who sees fish. Bring a water sample and a list of what changed recently. Honest limit: fish with heavy gill damage may die in the days after the water is fixed.

In the game

In Multiple Tank Syndrome, the Ammonia spike card is "a bacterial bloom after overfeeding". Just like real chemistry, ammonia is deadliest in warm or hard water: at 27°C or pH 7.5 and up the whole tank is lost; cooler, softer tanks lose half. A tank with fast-growing plants loses at most half. The Better filtration card saves the tank.