Aquaculture practice is progressing at a rapid pace and has already overtaken capture fisheries to meet the demand of the growing population. Disease has been the major constraint on the way to progress in this rapidly growing industry. Increasing stocking density has been the main focus for the overall increase of production and making aquaculture practice more profitable. Thereby, this industry often faces disease problems. However, the use of antibiotics and other chemicals for preventing or controlling the disease is not encouraged as it deteriorates the environment and develops drug-resistant strains. Biological agents such as probiotics, immunostimulants and bacteriophages have been used successfully to control different microbial diseases in aquaculture systems. All these products have been found to be safe in many aspects including the environment and human consumption. While controlling diseases in aquatic animals, these products also prevent the spread of pathogens to human beings and livestock. Research in many of these areas is still at the infant stage, and more effort is required to make these applications safe as well as economical. There are no strict distinctions between all these biologicals as far as their function is considered. All three biologicals mentioned can act as immunostimulants as they possess immunomodulatory properties. The limitations in the use of these products should be considered carefully, and effort should be made along those lines to make these products more suitable for the successful aquaculture practice.