A method of reducing the level of bacteria and viruses in a volume of water during aquaculture comprises: (a) providing the volume of water to be stocked with farmed aquatic organisms or eggs thereof; (b) prior to the stocking, introducing into the water an aqueous solution of glutaraldehyde in an amount such as to provide in the water from 0.1 to 2 ppm of glutaraldehyde; (c) stocking the water with the farmed aquatic organisms or eggs thereof at a time when the concentration of glutaraldehyde is 0.1 to 2 ppm. (d) allowing the farmed aquatic organisms or eggs thereof to grow; and (e) optionally during a period of the growth, introducing into the water at least one further portion of an aqueous solution of glutaraldehyde in an amount such as to maintain the concentration of the glutaraldehyde at 0.1 to 2 ppm.