are you looking to view the fish at night or simulate a moonlight like NewReefer asked.
if you want to view the fish, then i would suggest a red light. this part of the light spectrum is generally not seen by aquatic light. red gets filtered out by the water as you go deeper. this is why a lot of deep water creatures are bright red.
when doing a moomlight. you need to keep the brightness down. most people use way to much light for their moomlighting. the critters need a night time. remember even the moom rises and sets during the day and night.
I don't care to see the fish at night... I sleep too - haha. All I want is to simulate sunset and sunrise to some extent. Maybe a dim blue light that stays on for an hour after my main lighting goes off and comes on an hour before the main lights.