The lamps were hung at 60–80 cm above the aquaria. The same illumination intensity at the bottom of the aquaria was realized by adjusting the distance between the lamps and water surface and the number of light bulbs. The luminance at the bottom of experimental aquaria was 300±50 microE·m−2·s−1, which was measured by an underwater illumination photometer (JD-1A, made in Shanghai Xuelian Instruments Co.).