I know you're supposed to, but with my plants still only just recovering from their enforced 11 days of darkness, I was reluctant to keep the tank lights off for my new betta. In the end I kept the lights off for 2 hours and then turned them on again. The betta had caves and plants to shield his little eyes if necessary and didn't seem any worse the wear for it.
In short, I'd like to know the answer to your question too. I can't actually see how darkness is going to be good for the fish. What if it's nocturnal? It would seem like a neverending night!
