A method of treating water to reduce or prevent bacterial infection in an aquatic organism may include administering a particulate kaolin clay to the water in a dosage sufficient to reduce the presence of at least one undesirable bacterial species present in the water, wherein the particulate kaolin clay has at least one property selected from: (a) a BET surface area of at least about 25 m2/g； (b) a particle size distribution such that at least about 80% by weight of the particles of kaolin clay have an equivalent spherical diameter of less than 2 microns as measured by Sedigraph； and (c) a particle size distribution such that at least about 25% by weight of the particles of kaolin clay have an equivalent spherical diameter of less than 0.25 microns as measured by Sedigraph. The method may further include contacting an aquatic organism with the treated water.
