Algae, a diverse group of fast-growing photosynthetic primary producers, are considered to be important for sustainable aquatic ecosystems and have been used in different industries including aquaculture. Algae are thought to be a promising fishmeal for aquaculture due to their high protein and amino acid contents. However, aquaculture has some common challenges including insufficient and unsustainable quality diets for aquatic animals. Algae can be one of the ingredients of fishmeal due to their high-value nutrients like proteins, lipids and carbohydrates. Additionally, algae are also a good source of polyunsaturated fatty acids, pigments and vitamins. All these valuable substances are being used as feed additives in aquaculture to improve the growth-survival rate, skin colour, antioxidant ability and immune system of aquatic animals. This chapter explores the applications of algae in aquaculture, especially in fish farming. Recently, technological advancements have helped the efficient production of high-quality and low-cost algal biomass. Moreover, the role of fast-growing algal species on growth, development and other important aspects (including colour, immunity and disease resilience, etc.) of important fishes has also been discussed. In conclusion, the way forward is discussed where it appears that algae as aquafeed can be crucial for maintaining sustainable aquaculture.