Ammonia Poisoning in Aquarium Fish

Causes, Treatment, and Prevention

Fish tank filter

Jason English / Getty Images

Ammonia poisoning is one of the biggest killers of aquarium fish and it can often occur during the setup of a new tank. It can also occur in an established tank when too many fish are added at one time, when a filter fails due to power or mechanical failure, if bacterial colonies die off due to the use of medications, or there's a sudden change in water conditions. Elevated ammonia can't be seen, making regular monitoring a must so that it's not missed. Frequent water testing can detect levels of unionized ammonia (NH3) long before it turns into the invisible fish killer.

What Is Ammonia Poisoning?

Ammonia poisoning happens when a fish tank's ph levels become elevated, offsetting the nitrogen cycle. In ideal water conditions, ammonia levels should be nonexistent. However, tap water and the decomposition of organic matter inside the tank can both contribute to this condition.

Symptoms of Ammonia Poisoning in Fish

Ammonia poisoning can happen suddenly or over a few days. As the damage from ammonia poisoning continues, it will eventually cause damage to the brain, organs, and the central nervous system of a fish. You'll see the fish begin to hemorrhage, both internally and externally. Then, it will eventually die.


  • Gasping for breath
  • Loss of appetite
  • Lethargy
  • Red or purple gills
  • Bloody patches on the body
  • Laying at the bottom of the tank
Signs of Ammonia Poisoning in Fish

The Spruce / Alison Czinkota

Gasping for Breath

Initially, the fish might appear to be gasping at the surface for air.

Loss of Appetite and Lethargy

Your fish will start losing their appetites, as their bodily functions fail, and they will become increasingly lethargic.

Red or Purple Gills and Bloody Patches

The fish's gills will take on a red or lilac color, making them look like it's bleeding. As the problem progresses, the fish's tissues will begin to deteriorate, evidenced by red streaks or bloody patches on their body and fins caused by ammonia burns.

Laying at the Bottom of the Tank

In some cases, you might find fish laying at the bottom of the tank with clamped fins. A fish with clamped fins will hold its fins folded to its body without fanning them out, and the fish will become listless.

Causes of Ammonia Poisoning

Ammonia can enter the tank in several different ways:

  • Chemically treated tap water: Some water treatment companies use a chemical called chloramine—chlorine bonded to ammonia—as a more stable disinfectant for city water systems. Using tap water that's been treated with this chemical is a recipe for aquarium disaster.
  • Organic matter: The decomposition of organic matter—aquarium plants, fish excrement, and uneaten fish food due to overfeeding—is another way ammonia levels rise in tanks.
  • Bacteria buildup: If you do not do routine aquarium maintenance and cleaning, there will be a buildup of the bacteria that feed on this superfluous matter, resulting in an ammonia byproduct.
  • Fish byproducts: Fish, themselves, also contribute to rising ammonia levels in tanks. When a fish eats food, the protein-building process that ensues (in order for them to grow larger) can produce a byproduct that enters their blood. This results in the seepage of ammonia through their gills and into the tank.

Diagnosing Ammonia Poisoning in Fish

You will need to be on the lookout for any ammonia poisoning symptoms when setting up a new tank. One of the most obvious signs of ammonia poisoning is ammonia burns on a fish's body.


If the ammonia level in your tank rises above 1 ppm (part per million) on a standard test kit, begin treatment immediately by taking these steps:

  • Lowering the pH of the water will provide immediate relief, as will a 50 percent water change (be sure the water added is the same temperature as the aquarium). Several water changes within a short period of time may be required to drop the ammonia below 1 ppm.
  • If the fish appear to be severely distressed, use a chemical pH control product to neutralize the ammonia.
  • At this point, restrict feedings so that additional waste is reduced. In cases of very high ammonia levels, feedings need to be discontinued for several days.
  • Do not add any new fish should be added to​ the tank until both the ammonia and nitrite levels have fallen to zero.
  • If you have fish with ammonia burns, you may need to put them in a quarantine tank so they can be treated with quality antibiotic or antibacterial medication and the ammonia problem in the main tank is resolved.

Prognosis for Fish With Ammonia Poisoning

Even the smallest amount of ammonia can cause gill damage in fish and extremely high levels are oftentimes fatal. But if you can catch this problem very early in its progress and treat the water immediately, the fish can live normally. Fish treated for ammonia burns will respond to treatment within three to five days.

How to Prevent Ammonia Poisoning

Preventing ammonia poisoning involves a little bit of science to keep the water inhabitable. Start your fish off healthy when you are doing a set-up for a new tank:

  • Create good bacteria: When you start a new aquarium, ask a friend with a healthy, well-established aquarium for a cup of gravel from deep off the bottom of his or her aquarium. This dirty gravel is full of anaerobic bacteria that help complete the nitrogen cycle to keep both ammonia levels, nitrates, and other toxic byproducts at bay. In less than three weeks, the "good bacteria" contained in your friend's gravel will help the nitrogen cycle complete itself in your new tank. Without this established gravel, the process takes three to four months the traditional way. Take the following steps:
  1. Place one small cup of this unwashed matter into the very bottom of your new aquarium.
  2. Cover it with at least 2 inches of new aquarium gravel.
  3. Fill the tank with aged water.
  4. Abstain from adding fish to the tank until the cycle completes.
  • Avoid overstocking: That said, when starting a new tank, add only a couple of fish initially and do not add more until the tank has completely cycled. Even in a well-established tank, you should only add a couple of new fish at a time to avoid overstocking.
  • Feed fish appropriately: Another way to avoid ammonia poisoning is to feed fish small quantities of food, and then remove any food not consumed within five minutes.
  • Clean and change water: Clean the tank weekly, taking care to remove dead plants or other debris. Perform a partial water change at least every other week (more often in small, heavily-stocked tanks).
  • Test for ammonia: Test the water for ammonia at least twice a month to detect problems before they become serious.
If you suspect your pet is sick, call your vet immediately. For health-related questions, always consult your veterinarian, as they have examined your pet, know the pet's health history, and can make the best recommendations for your pet.
Article Sources
The Spruce Pets uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Aquarium Water Quality: Nitrogen CycleFlorida Department Of Agriculture And Consumer Services.