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An automatic flow-through dosing apparatus was used to determine the effect on Tilapia aurea of acute and chronic exposure to un-ionized ammonia. For fish not exposed to ammonia prior to acute testing, the 48-hour median lethal concentration (LC50) was 2.40 mg/liter un-ionized ammonia. After fish were exposed to sublethal concentrations of un-ionized ammonia (0.43–0.53 mg/liter) for 35 days, concentrations as high as 3.4 mg/liter caused no mortalities within 48 hours. Histopathological changes occurred in the gills of fish given both nonlethal and acute doses of ammonia. Capillary congestion, hemorrhaging, and telangiectasis were common symptoms of gill abnormalities.
Redner et al. (Sun,) studied this question.