The CARBODIET project proposes the formulation of a new functional feed, intended for the fattening of the fish species Solea senegalensis and Sophthalmus maximus, based on one of the diets formulated in the ALGADIET II project (Pleamar 2019) to which a biochar from biomass waste from the timber industry will be added. It has been widely demonstrated that in terrestrial animals the addition of biochars to feed leads to an improvement in their production. Although there are few trials in the literature regarding the use of these compounds in aquatic animal culture, they have been observed to reduce the excretion of nitrogenous compounds to the culture water and increase the protein content of fish. The aim of the CARBODIET project is to study in vivo the effect on two flatfish species of the addition of biochars to farmed feed, focusing on zootechnical, metabolic and animal health aspects, as well as on the quality of the farmed water.