NOVELTY - A method of controlling population of target pest microorganisms comprises exposure to an aquacidal compound comprising quinones, anthraquinones, quinine, warfarin, coumarins, amphotalide, cyclohexadiene-1,4-dione, phenidione, pirdone, sodium rhodizonate, apirulosin, thymoquinone or naphthalenediones (I).    USE - Useful for treating or sterilizing ballast water, and other water bodies, especially to prevent the inoculation of non-indigenous species through the ballast water released by shipping e.g. dinoflagellates, zebra mussels, vibrio cholera and vibrio fisheri. The composition is also useful for protecting marine or aquatic surfaces against mollusk (mussels, clams, or snails), bacterial, algal or bivalve infestation. Specifically useful against viruses, protists, fungi, molds, plants, holoplankton, meroplankton, demersal organisms, benthic organisms, biota, optionally encysted dinoflagellates, optionally encysted bacteria, protozoa, algae, pyrrophyta, cryptophyta, chrysophyta, porifera, platyhelminthes; psuedocoelomates, annelids, copepod larvae, ostracods, mysids, gammarids, decapod larvae, teleost fish larvae, bivalves e.g. Asiatic clams, and zebra mollusks.    ADVANTAGE - Relatively low concentrations of active compound are needed thereby minimizing their pollutant effect when the treated water is released.    DETAILED DESCRIPTION - A method of controlling population of target pest microorganisms comprises exposure to an aquacidal compound comprising quinones, anthraquinones, quinine, warfarin, coumarins, amphotalide, cyclohexadiene-1,4-dione, phenidione, pirdone, sodium rhodizonate, apirulosin, thymoquinone or naphthalenediones of formula (I):    R1, R5 = R6 or methyl;    R2 = H, methyl, sodium bisulfate, Cl, acetonyl, 3-methyl-2- butenyl or 2-oxypropyl;    R3 = R4, methyl, Cl, or 3-methyl-2-butenyl;    R4 = H or methoxy; and    R6 = H or OH.    An INDEPENDENT CLAIM is made for a method for killing mollusks pest using an aquacide comprising 2-methyl-5-hydroxy-1,4-naphthoquinone, 2-methyl-1,4-naphthalenedione, 2-methyl-2-sodium metabisulfite-1,4-naphthalenedione, 3-methyl-1,8-dihydroxyanthraquinone, and/or 2-methylanthraquinone.