One of the well-known field of the nano-product industry and service is fish farming, but sadly very little work is done on it. Materials used in the nanometer dimension retain very unique properties regarding absorption [1], [2]. Nanoparticles (NPs) maintain a highly distinctive set of characteristics, including high surface activity, low toxicity, good catalytic compatibility, and many more surface active centers [3]. Moreover, feed quality and nutrient bioavailability can be enhanced greatly by means of NPs [4], [5], [6].

Fish require energy for growth, reproduction, activity, and overall health in addition to fundamental metabolic processes [7]. This energy and a few other necessary elements are provided by the macronutrients, which include proteins, carbs, and fats [8]. Although they don't contain micronutrients like vitamins and minerals, they are nonetheless necessary for optimal functioning [9]. Trace elements like Se have great abilities to improve health indices of organisms by restoring enzymatic activity and reducing oxidative stress [10]. Selenium (Se) is considered as one of the crucial trace element because it has a very important role in promoting the health and productivity of animals. Livestock faces significant losses in case of insufficient Se levels. Se improves the different health parameters by improving the digestibility of nutrients in organisms, although a very little information is available about the effects of different sources of Se on the absorption of nutrients [11]. Se is very helpful in regulating the metabolism of thyroid hormone, antioxidant defense system and cell growth. It is also fundamental for improving the immunity, fertility and growth of animals [3], [12]. Deficiency of Se in diet may cause severe problems like infertility, oxidative diathesis, white muscle disease and pneumonia in animals [13]. Furthermore, decreased levels of Se might speedup the accumulation of reactive oxygen which can cause oxidative stress. The toxic effects of hydro-peroxides can be neutralized by using Se-dependent glutathione peroxides (GPx). The final weight and relative gain rate as well as the status of antioxidants as glutathione peroxides (GSH-Px) can be improved greatly by using Se NPs in the crucian carp basal diet [14].

The population of developing countries is increasing every day, leading to an increase in the demand of food with enhanced quality at an alarming level. The aquaculture sector however, is considered as a boosting industry to meet the increased demand for it, but it also needs proper nutrition for the fish. Even though aquaculture is a contemporary enterprise, a lot of work is still required to generate affordable, environmentally appropriate feed for the aquatic environment and the organisms that live there [15], [16].

Approximately 60% of the total value of a fish is used in fish feed [17]. Fish meal is one of the best sources of protein for fish feed as it holds the right amount of essential amino acids and all other required nutrients [18], [19] but sometimes it reaches out of the economic range. Researchers are working to reduce the price of fish feed without sacrificing quality throughout the world [20]. Following the same passion, we searched for the alternative source of fish meal which is cheaper than the regular meal. Sunflower meal was used as it is one of the best source for protein (45–48%) and readily available around the globe at a very low cost. For more than 20 years, scientists have used the by-products of these plants in fish feed [21], [22]. As a result of the above-mentioned characteristics, sunflower meal was chosen as a replacement for standard fish meal to produce fish feed for our study.

Fish like Indian major carps, of which C. mrigala is one, have very high market value due to their flavor and flesh quality. This group of fish contributes almost 67% of all freshwater fish products [23]. The current study aimed to evaluate the effects caused by Se NPs on growth and hematological parameters and how it effects the digestibility of nutrients in fingerlings and to assist the world in the production of eco-friendly and cost effective fish feed, as well as to improve the growth and hematological parameters, because these two are the key indicators that determine the health of the fish.