Ammonia is a substance toxic to all vertebrates and is found in the aquatic environment. While ammonia may be present in tap water, most of the ammonia found in an aquarium is produced by the fish. Ammonia is the primary waste metabolite produced by fish from the catabolism of protein contained within the feed. A second source of ammonia in the aquarium is decomposition of uneaten food, although this should be considered a relatively minor source. In the aquatic environment, ammonia exists in two forms in equilibrium: as unionised ammonia, NH₃, and as ionised ammonium, NH₄⁺. Thus, total ammonia concentration is the sum of the concentrations of un-ionised ammonia and ionised ammonium. The percentage distribution of each form is highly dependent upon the pH. Increasing pH results in an increase in the fraction of unionized ammonia (see graph).

Toxicity

The distribution of total ammonia between NH₃ and NH₄⁺ is important, because ammonia is considered to be toxic to fish, while ammonium is considered to be non-toxic at the levels experienced in aquaria. Most biological membranes are permeable to un-ionised ammonia and relatively impermeable to ionised ammonium (Randall & Tsui 2002). Because of this, ammonia present in the water either induces retention of ammonia in the fish, or the ammonia present in the water enters the fish via the gills by passive diffusion (Haywood 1983).
Acute ammonia toxicity affects the central nervous system of fish (Randall & Tsui 2002). While the exact nature of ammonia toxicity is not known in fish, it appears that ammonia interferes with physiological processes that eventually result in cell death in the brain. Another theory is that excessive ammonia depolarises muscle fibres and neurons, again leading to cell death (Randall & Tsui 2002).
Short-term exposures of fish to high concentrations of ammonia results in increased ventilation rate, hyperexcitability, erratic swimming, loss of equilibrium, convulsions and death (Smart 1976,1981, Haywood 1983, Russo & Thurston 1991). Chronic exposure to ammonia may lead to gill damage (swelling, mucus production, epithelial lifting, hyperplasia, break down of the pillar cell structure of the secondary lamellae, fusion of gill lamellae), ion imbalances, impaired liver function, impaired renal function, decreased food intake, growth and food conversion, and increased fin erosion (Larmoyeux & Piper, 1973, Smith & Piper, 1975, Smart, 1976, Alabaster & Lloyd, 1982, Haywood, 1983, Thurston et al., 1984, Tomasso 1994, Twitchen & Eddy, 1994).

Recommended levels

As with most water quality parameters, very little information is available on recommended levels of ammonia for tropical fish. Often a maximum level of 0.2 mg/l NH₄⁺ is applied for pH levels up to 8.0. At higher pH levels a maximum level of 0.1 mg/l NH₄⁺ is recommended.

How to avoid high ammonia levels

Several methods can be applied to avoid exposing the fish to high concentrations of ammonia:

1. Do not directly introduce fish to a new aquarium. The bacterial community in the filter needs time to develop. Also be aware that cleaning the filter or medicine use may severely affect the carrying capacity of a filter.
2. Don’t stock more fish than the filter is capable of dealing with.
3. Don’t overfeed your fish.
4. Use a sufficient amount of plants in your tank.

References

Alabaster, J.S., Lloyd, R., 1982. Water Quality Criteria for Freshwater Fish (2nd edition). Butterworth Scientific, London.
Haywood, G.P., 1983. Ammonia Toxicity in Teleost Fishes: a Review. Canadian Technical Report of Fisheries and Aquatic Sciences, 1177.
Larmoyeux, J.D., Piper, R.G., 1973. Effects of water reuse on rainbow trout in hatcheries. Progressive Fish-Culturist, 36, 2-8.
MacIntyre, C.M., Ellis, T., North, B.P., Turnbull, J.F., 2008. The influences of water quality on the welfare of farmed rainbow trout: a review. In: E.J. Branson (Ed.) Fish welfare, pp 150-184. Blackwell Publishing Ltd., Oxford
Randall, D.J., Tsui, T.K.N., 2002. Ammonia toxicity in fish. Marine Pollution Bulletin, 45, 17-23.
Russo, R.C., Thurston, R.V., 1991. Toxicity of ammonia, nitrite, and nitrate to fishes. In: D.E. Brune & J.R. Tomasso (Eds) Aquaculture and Water Quality, pp. 58-89. World Aquaculture Society, Baton Rouge.
Smart, G., 1976. The effect of ammonia exposure on gill structure of the rainbow trout (Salmo gairdneri) Journal of Fish Biology, 8, 471-475.
Smart, G.R., 1981. Aspects of water quality producing stress in intensive fish culture. In: A.D. Pickering (Ed.) Stress and Fish, pp. 277-93. Academic Press, London.
Smith, C.E., Piper, R.G., 1975. Lesions associated with chronic exposure to ammonia. In: W.E. Ribelin & G. Migaki (Eds) The Pathology of Fishes, pp. 497-514. University of Wisconsin Press, Wisconsin.
Thurston, R.V., Russo, R.C., Luedtke, R.J., Smith, C.E., Meyn, E.L., Chakoumakos, C., Wang, K.C., Brown, C.J.D., 1984. Chronic toxicity of ammonia to rainbow trout. Transactions of the American Fisheries Society, 113, 56-73.
Tomasso, J.R., 1994. Toxicity of nitrogenous wastes to aquaculture animals. Reviews in Fisheries Science, 2, 291-314.
Twitchen, I.D., Eddy, F.B., 1994. Effects of ammonia on sodium balance in juvenile rainbow trout Oncorhynchus mykiss Walbaum. Aquatic Toxicology, 30, 27-45