The present invention relates to the use of combinations of 4-bromo-2-(4-chloro-phenyl)-5-(trifluoromethyl)-1H-pyrrole-3-carbonitrile and an oxidizing agent, for eradicating, eliminating or reducing aquatic organisms in ballast water or bilge water whereby 4-bromo-2-(4-chloro-phenyl)-5-(trifluoromethyl)-1H-pyrrole-3-carbonitrile and the oxidizing agent are in respective proportions to provide a synergistic effect against fouling organisms. Suitable oxidizing agents are e.g. bromine, chlorine, ozone, sodium hypochlorite, chlorine dioxide, hydrogen peroxide, potassium permanganate, potassium ferrate, peroxydisulfates such as ammonium peroxydisulfate, sodium peroxydisulfate, and potassium peroxydisulfate peroxymonocarbonates such as calcium peroxycarbonate and sodium peroxycarbonate peroxydicarbonates such as sodium peroxydicarbonate and potassium peroxydicarbonate superoxides such as potassium superoxide and sodium superoxide peroxides such as potassium peroxide, and sodium peroxide and Fremys salts such as potassium nitrosodisulfonate, sodium nitrosodisulfonate, and disodium nitrosodisulfonate.
