Fish don't have eyelids, so I guess it would be best to turn it off sometimes to give them a rest None of my tanks have live plants, so I'm not really sure what the lighting requirements are for them, but I think if you don't have live plants, it's probably best to leave the lights off except for when you're in the room where the aquarium is. But if you leave them on all of the time, realize that it will promote algae growth and remember to provide plenty of places of shade for your fish to relax.