## 1 Introduction  
Aquaculture has drawn a great deal of attention during the last few decades due to the recognition on its potent role in meeting nutritional requirements, providing employment, and generating foreign trade [1]. Over the years, aquaculture practices have expanded into more diverse ways all across the globe. As a result, the incidence of infectious diseases in farmed fish has also been increased. In line with such situation, research efforts on defense mechanisms against microbial attacks in fish are increasing [2]. Various prophylactic measures (i.e., immunostimulation, vaccination, genetic improvement) and therapies (disinfectants and antibiotics) are in use [3]. As the application of antibiotics has become limited due to widespread incidences of antibiotic resistance and environmental pollution [4], immunostimulants have become promising alternatives for disease management in farmed fish [5]. A chemical, drug or action may be considered as an immunostimulant if it enhances the defense mechanisms or immune responses that improve resistance to diseases [6]. Both specific and non-specific defense mechanisms play crucial roles in control of microbial fish infections. Fish metabolism in particular depends on non-specific defense mechanisms [7]. Use of plant-derived immunostimulants is increasing in animals including fish nowadays due to fewer side effects, lack of residual problems, greater economic feasibility and ease of administration [8; 9]. Several studies have demonstrated an increase in various immunological parameters (e.g., respiratory burst, phagocytic complement and lysozyme activity) and plasma proteins (globulin and albumin) in different fish species after oral administration or intraperitoneal (IP) injection of plant-derived extracts [10]. Nitric oxide (NO) is an important molecule for regulation of immune function and has a direct antimicrobial effect [11]. Extracts of several herbs reportedly increased growth performance and improved immune response against bacterial infection in different fish species including _L. rohita_[12; 13; 14]. Natural metabolites produced by algae have been used for the production of bioenergy fuel [15] and removal of pollutants including heavy metals from the aquatic environment [16]. In addition, algal metabolites also have the potential to be used in medicine and pharmaceutics [17]. Fish diseases are one of the most serious constraints that reduce nearly half of the production, thus resulting in economic and food losses in aquaculture [18]. To control infectious diseases in aquaculture, health related measures should be addressed in a clinically proven and locally applicable manner [19]. Ethnoveternary therapeutic practices, nonspecific immunostimulants, vaccine, probiotics, and prebiotics are the most effective approaches for controlling infectious diseases of fish, apart from management practices, genetically resistant stock and water disinfection [20]. Plant constituents may act on receptors that trigger gene activation, which results in the production of antimicrobial molecules. Methanolic extract of _Chaetompha antenna_ has been reported to enhance the immunity of _L. rohita_ against _E. tarda_ infection [21]. _Chaetompha area_ extract is thought to possess several antimicrobial characteristics that may be associated with its immunostimulation properties [22] and has the potential to activate innate defense mechanisms of the fish, either directly or indirectly [23]. Dietary supplement fortified with _C. aerea_ extract has been reported to increase the growth of _L. rohita_ and increase resistance against bacterial infection caused by _Aeronomas hydrophilia_[24]. _L. rohita_ (Rohu) is one of South-East Asia's more important carp species of farmed fish due to its rapid growth and consumer preference [25]. _E. tarda_ causes Edwardsiellosis in several important species used in aquaculture, including chinoco salmon, eel, flounder, tilapia, and carps.