Credit for the photos: El-Sadig Arbab Hagar (Sudan), Samuel Simon Zigizo (South Sudan) and David Martin Sykora (Paraguay). Credit for the technical information: Ismael Radwan (project owner, Kafr El Sheikh, Egypt) Photo A-1 shows the whole biological filter for a 240-m3 closed aquaculture project. Photo A-2 shows the plastic rings which are the main substrate …