Ammonia Toxicity

All animals produce nitrogen compounds as a by-product of normal daily metabolism. Dogs and most other mammals produce urine which consists primarily of a compound known as urea, and most species of fish produce ammonia which is very toxic in the aquarium. Under normal circumstances, fish in the wild don't have ammonia toxicity problems since they live in millions or even billions of gallons of water and their ammonia becomes quickly diluted and incorporated into the nitrogen cycle where it is detoxified by naturally occurring bacteria.

High levels of ammonia in the aquarium is probably the number one killer of pet fish. Any measurable amount of ammonia indicates either an overloaded aquarium (too many fish or too much food) or an inadequate filter. Many aquarists suffer from a problem called "new tank syndrome." This problem arises because people place too many fish into an aquarium with an unestablished or inadequate biological filter.

Patience is key when beginning a new aquarium. A few hardy freshwater fish like tiger barbs can be used to start the nitrogen cycle in your aquarium. After four or five weeks, other compatible fish may be added to the aquarium.

Another common cause of elevated ammonia levels is a damaged or compromised biological filter. Since the biological filter consists of "good" bacteria which can be sensitive to certain antibiotics, all drugs should be used under the direction of a veterinarian familiar with your fish and aquarium. Antibiotics that are dumped into the water as a "shotgun" treatment can compromise a biological filter resulting in elevated ammonia levels.

Ammonia is also affected by the pH (dissolved hydrogen ions) of the water. When the pH is above neutral (greater than 7.0), it is primarily in the toxic or unionized form. If the pH is below 7.0, a significant portion of the ammonia will be ionized and not as toxic to the fish. Ammonia is especially harmful to marine fish since saltwater aquariums are almost always maintained at a pH of between 8.0 and 8.5.

Veterinary Care

Your veterinarian will be able to test your water and confirm whether or not you have an ammonia problem in your aquarium or pond. If an ammonia problem is identified, then she or he will, combined with your input, identify the source of the problem and make efforts to correct it. In most cases, treatment will be supportive (clean water, plenty of oxygen, good food), and your fish will begin feeling better once the ammonia levels have been reduced to an acceptable level.

Home Care

An elevated ammonia level needs to be dealt with immediately in the form of water changes or physically removing the fish to an aquarium or other container with clean water. If the ammonia levels are high (greater than 2.0 parts per million) then you may have to perform daily 25-50 percent water changes. Remember that if you are using municipal water, it will likely contain chlorine or chloramine. These chemicals are added to the water to make it safe for human use but they are very toxic to pet fish.

Always use a water conditioner that is labeled to neutralize chlorine and chloramine before adding new water to an aquarium or pond. These water conditioners are available in most pet stores.

Preventative Care

Prevention is the best solution for ammonia poisoning. By not overstocking, overfeeding, or overmedicating your aquarium, the chances of an ammonia problem are greatly reduced. Coupling these strategies with an adequately functioning biological filter will insure that ammonia toxicity will be something you only read about.

Ammonia is the primary nitrogenous (protein breakdown) waste product excreted by fish and is also produced when organic matter, including uneaten food, decomposes in water. Ammonia is generally either in the toxic unionized form or the non-toxic ionized form. The ratio between the two compounds depends on temperature, pressure, salinity, and most importantly, pH (number of hydrogen ions dissolved in the water). Generally, the higher the pH, the more unionized (harmful) ammonia is present. The total ammonia nitrogen (TAN) reading represents both forms. A TAN measurement of 3.0 parts per million (ppm) would be deadly at a pH of 8.5 but relatively harmless at a pH of 6.0. Hobbyists and professionals alike commonly ask at what point should ammonia levels be considered dangerous? The best answer is that any detectable ammonia in an established aquarium indicates a filtering deficiency. Either the filter is inadequate for the system or the biological load is too great for the filter.

Affected aquarium or pond fish will frequently be anorexic (not eating), lethargic (slow or depressed), and may have "clamped fins." They may also swim erratically, and can be found "piping" at the surface for air or bolting from one side of the aquarium to the other. Some fish produce excessive mucus in response to the elevated ammonia and may appear cloudy or pale. The eyes may be opaque and the gills pale or swollen. The aquarium or pond water is frequently cloudy and in some cases dead fish and uneaten food may be present (contributing to the problem). Morbidity (illness) and mortality (death) can be quite high. Frequently, the aquarium or pond may have been recently established. This situation is commonly referred to as "New Tank Syndrome." If ammonia toxicity occurs in a mature system, new fishes may have been added without increasing filtration. Another common finding in the medical history is a change in the management of the system (a new person is feeding the fish, the type and amount of food has changed, the frequency of water changes has decreased, dead plants and fish are not removed immediately).

The exact physiological mechanism of ammonia toxicity in fishes is unknown. We do know that when aqueous ammonia levels are elevated, blood and tissue levels of ammonia rise. This situation may lead to decreased oxygen transport by the blood, an increase in blood pH, and an osmoregulatory disturbance for the fish.

Veterinary care should include diagnostic tests and subsequent treatment recommendations.


There are several different types of biological filters. The first type is the popular undergravel filter. Most utilize a plastic grid which lies at the bottom of the tank allowing for a small water space beneath it. Several centimeters of gravel are placed over a porous plate. Aerated water is then pulled through the gravel bed via air lift tubes that are attached to the plastic grid. The nitrifying bacteria colonize the gravel, and the aquarium water is literally pulled through the filter exposing the bacteria to the nitrogen compounds in the water.

A second type of biological filter is termed a wet/dry filter and may also be called an ammonia tower or trickle filter. With these filters, the bacterial bed is not submerged in water, but rather is sprayed with aerated water which passes through the filter bed by means of gravity. These filters are desirable since they tend to allow for a large surface area, and consequently many bacteria can colonize the filter, increasing overall efficiency. In recent years, "bubble bead" and fluidized sand filters have been introduced to the aquarium hobby, both of which function as highly efficient biological filters.


The prognosis for affected fish depends on a variety of factors including ammonia levels, duration of exposure, and species affected. In many cases, ammonia toxicity is a chronic condition with high morbidity and low mortality. If the problem is addressed and corrected before fish begin to die, the prognosis is generally favorable.


The pet fish should be monitored closely for clinical improvement and the water retested on a regular (at least every day for a week) basis in order to document and monitor decreasing ammonia levels.