**1. Introduction**

Diet supplementation is an important aspect of aquaculture management, especially in intensive or semi-intensive fish culture, and is promising for increasing fish production [1]. Protein, on the other hand, is required for normal tissue function, fish body protein maintenance and replenishment, and growth. Due to the high cost of protein, it is more cost-effective to use all of the protein for tissue repair and growth rather than catabolizing it for energy [2].

This cost is determined by a variety of factors, including the amount of protein in the product, the source and kind of ingredients sourced from plant or animal sources, and manufacturing techniques. Different feeding management solutions, such as on-demand feeding regimes, are in addition to generating low-cost diets and/or good husbandry and pond management [3] possibly aid in the growth of fish. One issue that fish farmers face is finding a balance between rapid fish growth and the most efficient use of available feed. Because managing their feed intake in accordance with their energy needs is expected to improve when fish are fed with a proper feeding frequency, growth and feed conversion ratio are expected to improve.

The concept of a feeding schedule was created to account for fluctuations in the protein requirements and digestibility of farmed fish. In terms of practicality, the

ideal condition would maximize the utilization of dietary protein for growth while reducing the usage of proteins for functional protein synthesis, gluconeogenesis, lipogenesis, and energy production [4].

Algae are photosynthetic creatures that provide the ultimate source of cellular carbon as well as chemical energy to other organisms. As a result, they were frequently referred to as primary producers. Microalgae (seaweed) and macroalgae (seaweed) are the two types of algae (unicellular). Microalgae require light, carbon dioxide, and nutrients to grow. Microalgae are grown for food, to produce valuable compounds, as biofilters to remove nutrients and other contaminants from wastewater, in the cosmetic and pharmaceutical industries, and for aquaculture. Also, due to their high oil content and quick biomass production, microalgae could be a viable source of biofuel [5].

Because the fish expends energy to collect prey but gains no energy from ingesting it, some plankton pass through the gut of planktivorous fishes undigested. In this circumstance, the fish may detect and reject such undesirable creatures. In the early stages of a fish's life, natural food is essential [6].

Cyanobacteria have been used in photosynthesis and its genetic control, photoregulation of genetic expression, cell differentiation and N2fixation, nitrogen, carbon, and hydrogen metabolism, resistance to environmental stress, and molecular evolution due to their benefits for humans, animals, and the environment (photosynthesis) [7]. When carp are removed, algal output decreases due to nutrient depletion, macrophytes grow due to reduced turbidity, and zooplankton increases due to increased macrophyte cover.

*Spirulina* is a cyanobacterium that has been commercially produced for over 10 years because of its high nutritional content, which includes protein, amino acids, vitamins, minerals, vital fatty acids, and carotene [8]. *Spirulina* can be used as a nutritional supplement for people, as well as a feed supplement for animals with economic benefits. For example, when fed to trout, sea bass, fancy carp, red tilapia, shrimp, and mollusk, it can be a good food supplement*.* The alga has been discovered to be a good source of protein as well as a way to improve the color, flavor, and quality of meat [9].

*Spirulina* has been shown to have therapeutic effects in animals, including fish, as a growth promoter, probiotic, and immune system booster [10]. *Spirulina* is used to help livestock, poultry, prawns, carp, canaries, and exotic birds develop faster [11]. Preclinical research indicates *Spirulina* possesses antimutagenic, hypocholesterolemic, immunological, and antiviral effects.

*Spirulina*'s chlorophyll functions as a purifying and cleansing agent against harmful chemicals. It is also utilized as a probiotic agent and as a food supplement to increase color in ornamental fish. *Spirulina* contains protein (60–70%), necessary amino acids and fatty acids, phycocyanin (14%), chlorophyll (1%) and carotenoid colors (0.37%), vitamin B-12, and minerals that play key functions in animals in a variety of ways [10, 12].

*Spirulina* is a source of linolenic acid (GLA), an important fatty acid with therapeutic effects. Iron, calcium, chromium, copper, magnesium, manganese, phosphorus, potassium, sodium, and zinc are among the minerals found in *Spirulina*. By breaking down indigestible feed components, *Spirulina* promotes the intestinal flora of fish, according to Bargey's Manual of Determinative Bacteriology, *Spirulina* is an oxygenic photosynthetic bacterium that belongs to the Cyanobacteria and Prochlorales families. In this classification, the sequence of the rRNA subunit 16S is considered. In 1989, these microorganisms were classified into two genera, according to the suggestion by Gomont in 1892 [12].

*Spirulina* is becoming a popular health food all over the world. It is a filamentous Cyanobacterium that belongs to the Cyanophyte class of algae. Furthermore, *Spirulina* is a natural resource that is high in protein, amino acids, vitamins, minerals, essential fatty acids, B-complex, and -carotene [13].

*Spirulina* has been shown to be capable of breaking down indigestible feed components and improving the intestinal flora in fish in previous studies [12]. In fish, the creation and release of enzymes that transfer lipids for growth rather than storage. Furthermore, the -carotene in *Spirulina* helps to keep the mucous membrane in place, preventing hazardous materials from entering the body. *Spirulina*'s chlorophyll functions as a purifying and cleansing agent against harmful chemicals [14].

Phosphorus and nitrogen from agricultural and industrial effluents, as well as home wastewater, can produce major eutrophication in any aquatic body. These nutrients, on the other hand, can be used to boost plant growth, such as phytoplankton, which can be used as natural fish food or in pharmaceuticals. Due to its great nutritional content, *Spirulina* is one of the most promising microalgae for culture [15].
