An outbreak of gnathiids in a captive fish population in a large exhibition aquarium is described, including a method for enumeration of the causative agent. This procedure for harvesting and counting the parasites was based on the positive phototropism of their larvae (zuphea and praniza). Water pumps with spotlights allowed capture of larva using a 200-micron plankton net. Harvested samples were rinsed with fresh water in the laboratory to immobilise the zuphea and praniza and thus facilitate the enumeration process by direct observation under a stereoscopic glass. The sampling method was found to be representative of the degree of infestation experienced by the entire aquarium system; this finding is very important as it allows the enumeration of different life-cycle stages (zuphea, praniza and adults). The outbreak was studied and the infestation monitored over a period of many years. Several substrates were assayed for the capture of gnathiid males, with identification of the causative parasite by such morphological characteristics as Gnathia maxillaris. Physical and chemical treatments were applied to mitigate the infestation. Selected to control the infestation and its lifetime in closed marine-water systems, the Trichlorfon was evaluated by chromatography.