DRUG FOR FISH PARASITE

PURPOSE: To obtain a medicine for preventing parasites on fishes, especially parasitic protozoan and metazoa by including a substituted 1,2,4-triazinediones or a salt thereof. CONSTITUTION: This medicine for preventing parasites on fishes includes a compound of the formula [R is an aromatic group or a heterocyclic aromatic group bonded by C; X is O, S, SO, SO2 or C(CN)H; R is H, a halogen, NO2 , an alkyl, an alkoxy, a halogenoalkyl or an halogenoalkoxy; R is H, an alkyl, an alkenyl, an alkynyl or an aralkyl] or a base salt thereof, e.g. 2-chloro-α-(4- chlorophenyl)-4-(4,5-dihydro-3,5-dioxo-1,2,4-triazin-2(3H)-yl)-phenyla cetonitrile. The treatment by the medicine is performed by an addition to a food, a short period treatment by a transcutaneous bath (the concentration is 5-10mg/L) when transferring the fishes between feeding vessels or a temporary or permanent pool treatment (the concentration is 0.1-5mg/L).