It depends on what type of algae you are using. Caulerpa should be lit 24/7 to prevent it from going sexual. That can lead to water quality issues and a possible tank crash. If you use Cheato, etc you can just light it opposite the tank. In my experience, and from others I've talked to, lighting the fuge opposite the display tank sounds good but really doesn't do all that much to stabilize PH overnight. I prefer running the lighting as much as possible in order to maximize macro algae growth and nutrient export. I give my fuge 4-5 hours darkness daily just to give the animals (pods
, worms, etc.) a dark period to mimic nature. Some animals seem to prefer to spawn after lights out.