FinFettle

Ammonia Poisoning in Fish: Signs, Causes and What to Do

Quick answer

Ammonia poisoning happens when ammonia builds up in the water and burns your fish's gills — most often in a new, uncycled tank. Signs include gasping at the surface, red or inflamed gills, lethargy and loss of appetite. The immediate fix is a large dechlorinated water change to dilute the ammonia, plus cutting back feeding, and testing daily until it holds at 0 ppm.

By Haitham Houly · Updated October 9, 2026

Flat illustration of a fish gasping at the surface with an inflamed red gill patch, a water-change jug refilling the tank, and a test tube reading high against an ammonia colour chart
Illustration

If fish in a newish tank are gasping at the surface, sitting listlessly, showing red or inflamed gills, or dying for no visible reason, ammonia poisoning is one of the first things to rule out. It's among the most common killers of beginner fish, and because ammonia is invisible in water, it's easy to miss until a test kit reveals it.

The immediate move is simple: if a test confirms ammonia, do a large dechlorinated water change now to dilute it, cut back feeding, and keep testing daily. The rest of this page explains how to recognise ammonia poisoning, why it happens, and how to stop it coming back.

Is it actually ammonia poisoning?

Ammonia poisoning shows up as a cluster of stress signs rather than one tell-tale mark:

  • Gasping at the surface or near filter outflow (ammonia damages gills, so oxygen uptake suffers).
  • Red, purple or inflamed gills; sometimes red streaks or patches on the body or fins.
  • Lethargy — sitting on the bottom, clamped fins, hiding.
  • Loss of appetite.
  • In a new tank, fish dying with no spots, fuzz or obvious disease.

The context matters as much as the symptoms. If the tank is new, was recently overstocked, or you've had a filter problem, ammonia is a prime suspect. The only way to be sure is to test — ammonia doesn't announce itself any other way. Keeping a betta? See the betta-specific new tank syndrome guide for signs and timing particular to bettas.

Total ammonia vs. the toxic form

Your test kit measures total ammonia. But only part of that — the "un-ionised" fraction — is the highly toxic form, and its proportion rises with higher pH and temperature. University of Florida and Texas A&M Extension both note that the same total reading is more dangerous in warm, alkaline water. Practically: treat any detectable ammonia as something to fix, and be extra cautious in a warm, high-pH tank. My aquarium ammonia guide explains what a reading means in more detail.

Check the water first — this is the diagnosis

For ammonia poisoning, testing is the diagnosis. Use a liquid kit to check ammonia, nitrite and nitrate:

  • Ammonia above 0 ppm → ammonia poisoning is in play.
  • Nitrite also up → the tank is mid-cycle; see new tank syndrome.
  • Both 0, nitrate present → the tank is cycled and ammonia isn't your problem; look elsewhere.

Free tool

Water changes work by dilution, so knowing your tank's true water volume matters. This calculator gives you the real figure after glass, substrate and decor — so a '50% change' is actually 50%.

Aquarium volume calculator →
Liquid ammonia test tube held against the API colour chart, showing a low yellow-green result
Reading an ammonia test: hold the tube against the chart. Yellow is the low, safe end of the ammonia scale — this one's sitting right down near the bottom. The greener (then bluer) it turns, the more ammonia your fish are sitting in.

What to do next (husbandry-first)

The plan is about removing ammonia and protecting the fish while the biological filter catches up — not medicating.

Do a large water change immediately. Change 25–50% with dechlorinated, temperature-matched water. This dilutes the ammonia straight away and is the single most effective thing you can do. Repeat if the reading is still high after testing.

Always dechlorinate. Tap-water chlorine and chloramine harm fish and kill the beneficial bacteria you need. A conditioner that also detoxifies ammonia gives you a bit of breathing room between changes — useful, but not a substitute for the water change itself.

Cut feeding right back. Less food means less waste means less ammonia. Feeding lightly for a couple of weeks won't harm healthy fish.

Stop adding fish, and don't clean the filter. More fish means more ammonia; deep-cleaning the filter removes the bacteria that will eventually solve the problem.

Test daily. Keep ammonia and nitrite below about 0.25 ppm with water changes until the tank finishes cycling and holds them at 0 on its own.

When it's beyond a water change

Ammonia burns gills, and severe or prolonged exposure causes damage that water changes alone won't reverse. If fish are collapsing quickly, bleeding from the gills, or not improving despite clean water, get them out of the toxic water: the best move is into mature, cycled water — a spare tank if you run one, a friend in the hobby, or your local shop (many will hold or rescue a struggling fish). This page is educational and husbandry-first, not a diagnosis.

How to prevent it

Ammonia poisoning is really a cycling problem, so the prevention is the same: cycle the tank properly. Whether you go fishless or run a carefully-managed fish-in cycle (a few hardy fish, daily testing, water changes as needed), the goal is the same — enough filter bacteria that the ammonia your fish produce is processed almost as fast as it appears. My cycling guide covers the method, and seeding the filter with mature media from an established tank is the fastest head start. A bottled bacteria starter such as Fluval Cycle can help too, though it's not a substitute for testing. Running a fishless cycle instead? Dr Tim's Ammonia covers how much to dose and what to expect as the readings move. After that, stock gradually and don't overfeed, and you shouldn't see ammonia again.

Common questions

What are the symptoms of ammonia poisoning in fish?
Common signs are gasping at the surface, red or purple inflamed gills, clamped fins, lethargy or sitting on the bottom, loss of appetite, and sometimes red streaks on the body. In a new tank, fish dying with no spots or fungus is often ammonia poisoning. These are effects of gill damage and stress, not a contagious disease.
How do I lower ammonia in my aquarium fast?
A large dechlorinated, temperature-matched water change is the fastest safe way to dilute ammonia — 25–50% now, repeated if the level is still high. Stop or cut feeding, and don't add more fish. A water conditioner that detoxifies ammonia can buy a safety margin between changes, but water changes are the real lever.
What level of ammonia is safe for fish?
The target is 0 ppm. Any persistent reading above 0 is a problem. Ammonia's toxicity rises with higher pH and temperature — the same total reading is more dangerous in warm, alkaline water — so even a 'low' number can matter. Treat any detectable ammonia in a stocked tank as something to correct.
Why does my new tank have high ammonia?
Because it isn't cycled yet. A new tank hasn't grown the bacteria that convert ammonia to less harmful compounds, so the ammonia your fish produce has nowhere to go and accumulates. This is new tank syndrome. The long-term fix is to let the tank cycle while protecting the fish with water changes.
Can fish recover from ammonia poisoning?
Often yes, if you act quickly and keep the water clean while they heal. But sustained ammonia exposure causes lasting gill damage and leaves fish vulnerable to infection, so recovery isn't guaranteed for badly affected fish. The sooner ammonia comes down and stays down, the better their chances.
What does an ammonia burn look like on fish?
Ammonia burn shows up on the gills first: they turn red, purple or visibly inflamed instead of their normal healthy pink, sometimes with a ragged or frayed edge. You may also see red streaks on the fins or body and faster, more laboured breathing. It's chemical gill damage, not an infection, so it won't spread between fish — but a burned fish is more vulnerable to one afterwards.

Sources

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