Two bamboo platforms (1.2m wide and 21m long) were constructed 30cm above fish pond water surface and 9 wooden trays (l × w × h=100cm×100cm×12cm) which were lined with polythene sheets were placed on each platform. Three trays whichwere randomly selected from each platform were filled with gravel, another 3 trays with sand and the rest with styrofoam sheets. The sand and gravel were washed to remove contaminants and placed in the trays at a depth of 10cm. Each tray (a hydroponic unit) was fitted with a 20mm PVC pipe and a control valve for irrigation.
Lettuce seeds (Chia Tai Co. Ltd, Bangkok, Thailand, Aeroplane Brand; black-seeded simpson cultivar) were sown in four plastic trays filled with loam soil (2cm in depth). Plants were thinned after 3daysofgerminationandtwenty-onedayoldseedlingsweretransplanted into the experimental trays at a density of 12 plants per tray (25cm apart from each other). Hydroponic units were irrigated twice daily at 0800 and 1600h using either filtered or unfiltered pond water according to the assigned treatments. The flow rate was adjusted to a 0.05Ls−1 to supply 90L of water per irrigation. Irrigated water was retained in the hydroponic units for 45min and then allowed to drain back into the fishpond. The vegetable hydroponic culture was started 4 months after stocking catfish to allow for nutrient accumulation in the pond. The experiment was conducted for 54 days.