2. Materials and methods
The experiment had a factorial arrangement of 6 treatments,
namely, 3 feeding frequencies for red tilapia Oreochromis sp. X 2
photoperiods for water spinach I. aquatica Forsk. Illumination was
12 h or 24 h daily. An equal daily rationwas evenly fed to the fish 2,
4 or 6 times at 12-h, 6-h and 4-h interval, respectively. Each
treatment had 2 replicates or experimental units. The experiment
was completed in 4 weeks.
Each experiment unit had an orange plastic tank (115 cm L
102 cm W 99 cm H), filled with 1000 L freshwater and stocked
with 8 fish at 467 30 g each or around 3.7 kg m3. Constant
aeration was provided at tank bottom by a membrane disc diffuser
(LTD-325/325 mm, Aquatek, Kaohsiung, Taiwan), which had a
membrane diameter of 32.5 cm and provided an intensive air
throughput of 0.02e0.12 CMM 1e3 mm diameter air bobbles. A
piece of 3-cm thick polyethylene raft covered almost entire water
surface except a 15 cm 15 cm corner cut open, allowing an
automatic feeder release pellet feed into thewater. A cut plant stem
25.1 3.7 cm or 7.8 0.5 g was wrapped around with layers of
sponge, stuffed in a black plastic ring (4.5-cm D) then fit into one of
the 63 evenly distributed round holes. Total plant biomass on a raft
was 490.2 5.5 g. Part of a stem was submerged to expose its first
bottom node, allowing for root initiation. A piece of coarse screen
(2.54 cm mesh) was secured 20 cm below the polyethylene raft to
prevent the plant root from possible disturbance by the fish. Each
tank was encased in a 200-cm tall wooden framework, which a
timer, a feeder and an illumination device could be fixed onto. A
near-sunlight 28-W 115-cm T5 tube was used for illumination,
hanging 25 cm above plant top and its height was adjusted as the