A composition for treating waters, e.g. ballast water or injection water for oil recovery, to kill in-situ aquatic invasive species comprises at least one biocide capable of killing both animal and plant micro-organisms. The at least one biocide preferably comprises Brilliant Green, Gentian Violet, and/or erythrosine, and a wetting agent or detergent-like compound such as CTAB or CTAC. The invention also relates to a system for treating ballast water in situ comprising means for injecting a composition for treating ballast water means for measuring the flow rate or amount of ballast water to be treated means for controlling the dosing of the composition and means for storing or receiving the composition. The invention also relates to a method of detecting viable aquatic organisms in ballast water in situ comprising detecting metabolism in viable micro-organisms in ballast water and, therefore, measuring the efficacy of any treatment.
