Method of controlling fish parasites

The application relates to the use of compounds of formula (I) wherein A is an unsubstituted or mono- to tetrasubstituted, aromatic or non-aromatic, monocyclic or bicyclic heterocyclic radical, where one to two of the substituents of A can be selected from the group consisting of halo-C1-C3alkyl, cyclopropyl, halocyclopropyl, C2-C3alkenyl, C2-C3alkynyl, halo-C2-C3alkenyl, halo-C2-C3alkynyl, halo-C1-C3alkoxy, C1-C3alkylthio, halo-C1-C3alkylthio, allyloxy, propargyloxy, allylthio, propargylthio, haloallyloxy, haloallylthio, cyano and nitro, and one to four of the substituents of A can be selected from the group consisting of C1-C3alkyl, C1-C3alkoxy and halogen; R is hydrogen, C1-C6alkyl, phenyl-C1-C4alkyl, C3-C6cycloalkyl, C2-C6alkenyl or C2-C6alkynyl; and X is N-NO2 or N-CN, in the free form or in salt form or, if appropriate, tautomers thereof, in the free form or in salt form, for controlling fish parasites, in particular sea lice.