My goldfish have recently begun sleeping on the bottom of their tank no matter the time of day or night for long periods of time. Yesterday one died..had been feeding and foraging in morning then dead? I've tested the water for everything all is good. The plants are healthy. no algae..clean tank. 50 gal for now two goldies about 4" long. Optimum temp. Tank and fish are about three years old. Light runs about 6 hrs/day. Supplemented by the natural light in my apartment.