The experimental coupled aquaponic system used in this trial is shown in Figure 1. In brief, the system was composed of three replicate aquaponic units within a greenhouse, being each aquaponic unit an experimental replicate. Each aquaponic unit consisted of a 2,000 L tank for holding fish, followed by a sedimentation tank where suspended particles like uneaten pellets and fish faeces settled down and were removed. Then, water flew into a 125 L biological filter tank filled with bio-balls, where nitrifying bacteria converted ammonia into nitrites and nitrates, adapted from IRTAmar® system. Water flew by gravity into the hydroponic unit where lettuces were supported on polystyrene rafts that covered the entire surface. Strong aeration was provided by a 0.3 Kw h air blower to ensure sufficient oxygen for the growth of plant roots. Finally, water was pumped (2,470 L/h; 0.1 Kw h) from the hydroponic unit 101 back to the fish-rearing tank through a submerged water pump (SERA Pond Precision SP2000, SERA GmbH, Germany). The total volume of each aquaponic unit is 7.1 m3, with a water turnover time for an entire circulation cycle of ca. 3 hours. Water and air temperatures were ambient since no thermal control was provided neither for the aquaponic water nor the greenhouse air.
