Excerpt from Sodium Cyanide as a Fish Poison: Special Scientific Report Fisheries No; 253 Fish were removed from the holding tanks and placed in an aquarium containing the diluent 10 to 15 minutes before the addition of the desired concentration of poison. If 100 percent mortality did not occur within 72 hours the tests were terminated. Continuous observa tions of the behavior of the fishes were made for 2 or 3 hours and if complete mortality had not occurred within this period, further observa tions were made at irregular intervals as necessary. When there was no response to gentle prodding, the fishes were considered to be dead and the time required to produce 100 percent mortality was recorded to the nearest one-tenth of an hour