I had a similar problem with my first tank. In this case it was because of hair algae dropping the oxygen level at night. O2 saturation in saltwater is only about 7 ppm, as opposed to about 13-14 ppm in FW. Fish start to crash at anything less about 5 ppm. That's a pretty narrow window. It would make sense that fish die-off would occur at night when there is no oxygen being produced by the algae in the tank.

Gary