In my professional practices I have been working in a research project, called AKURA, for the development of sustainable aquaculture of mullet fish in the Basque Country (Spain). The aim of the professional practices was to test a recirculating aquaculture system in order to increase the sustainability of the process. Gaiker was the company (Research Centre) responsible of developing the RAS system. It was made up of nitrifying bacteria forming a biofilm around plastic carriers swimming free in the biofilter.