Abstract: Crustacea, and especially shrimps are farmed in ponds, which can become contaminated with pathogenic bacteria, and in which predatory fish can consume a significant proportion of the shrimps if not controlled. The ponds desirably should contain a controlled level of phytoplankton. Such ponds can be treated with dilute peracid such as peracetic acid at a concentration that is significantly below the concentration which is lethal to the shrimps in order to control phytoplankton and/or pathogenic bacteria and to kill the fish. The peracid is preferably selected in the range of from about 0.5 to about 5 mg/l peracetic acid.