Hi before you do the lights out bit, try to harvest as much as you can, scrape the glass, pull the stuff off the rocks etc. When you cover the tank, I would point the power heads a bit more down wards in the hopes of blowing the dying algae off of the rocks, into the water coloum and then down your over flow. So hopefully the algae won't make too much of a mess. When it is time for lights on, be ready to do a water change to try to get some of the crap out of the water from the dying algae. If at lights on the algae is still there you could try to brush the rocks with a tooth brush to get some more off. It should be a bit easier to get off. If there is still quite a bit of algae do the lights off bit again. The fish and the snails will not be bothered by a period of darkness. As for the corals, it will depend on what kind you have. I did this and my soft corals were OK.