In a global context of a fast growing aquaculture, the reduction of the aquatic pollution through the optimisation of aquafeeds and their nutritional value is a major concern for farmers, researchers and governments. More than 50% of phosphorus (P) reserves in seeds of most legumes are stored as phytate-bound P which is not bioavailable for monogastric animals including most fish species. Therefore, the presence of phytate in the aquafeed is traduced directly or indirectly in negative effects on growth performance and on minerals, protein and energy utilisation by fish. Phytases have been used in poultry and pig productions during the last decades, mainly to reduce the environmental impact and P loads from farm effluents, as well as to reduce feeding costs through nutrient sparing. Moreover, in recent years, researchers and companies are paying increasing attention to the use of phytases in high doses to obtain extra-phosphoric benefits on dietary nutrient utilisation and growth performance in monogastrics. The use of phytases in aquafeeds is still in an initial stage, but it is presented as one of the most effective tools used by the aquaculture to include alternative plant protein ingredients in diet formulas, improve fish growth and control the diet-related environment pollution from aquaculture operations. This chapter presents a discussion about the intervening factors that influence the potential benefits of phytases in fish plant based diets and how the inclusion of this additive might improve their protein and P bioavailability. The properties of phytic acid under the gastrointestinal environment, its interaction with other minerals, nutrients and digestive enzymes, as well as the use of phytases to avoid the negative effects of phytate on nutrients digestibility and water pollution are also discussed.