The most active time for nocturnal fishes is during the early evening and morning hours, when they feed. Grouper, lionfish and eels are among the species that are active mostly at night. You can keep a nocturnal tank by creating a cave-like environment and keeping the tank away from a bright window or area. Use coral rocks and try to set up the hiding spaces so you still may observe your pets under low light.
For more information, please read the story Night Life on the Reef.