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Ammonia Toxicity

By: Dr. Greg Lewbart

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Veterinary care should include diagnostic tests and subsequent treatment recommendations.

  • A number of clinical conditions of ornamental fishes resemble ammonia toxicity. Diseases to rule out would include nitrite toxicity, hypoxia, ectoparasites, and bacterial gill disease. Skin and gill biopsies combined with a thorough water evaluation will help your veterinarian narrow the list of differentials.


  • Nearly all commercial water test kits measure ammonia in the water. Most of these test kits measure the TAN. More accurate measurements can be made with ion-specific electrodes but this is not necessary for quantifying and identifying an ammonia problem.


  • Once an ammonia problem has been identified, the very first step is to initiate water changes to dilute the ammonia. Most freshwater and marine fish will tolerate a 50% water change every 12-24 hours to reduce toxic ammonia levels. Marine fish are exquisitely more sensitive to ammonia since the pH in a saltwater aquarium is usually about 8.3 while most freshwater systems are close to neutral (7.0). Consequently, a greater percentage of the TAN is in the toxic, unionized form in seawater. Chemical filtration materials that adsorb (hold onto) ammonia, such as zeolite and activated carbon, can help reduce ammonia levels but frequent water changes combined with the establishment of a biological filter are crucial to resolving the problem.

  • The biological filter is the best and most efficient means of removing ammonia from an aquatic system. This type of filter utilizes nitrification, a natural process which occurs constantly in soil and water. Nitrification involves the conversion of ammonia to nitrate in a two step process. Nitrosomonas bacteria oxidize (chemically change) ammonia to nitrite, and nitrobacter bacteria oxidize nitrite to nitrate. Stable populations of these bacteria must exist in the filter for the nitrification process to perform efficiently. These bacteria require oxygen and ammonia as a nutrient source. Under typical conditions, it takes several weeks for the filter to develop and function adequately. When an aquarist attempts to "rush" the biological filter by loading a tank with fish before the filter is established,he or she will likely be confronted with the "New Tank Syndrome."

    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.

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